"IO" -> "System".

Get rid of the separate opt-in IO class and replace it with a core
System class.

- Remove wren_io.c, wren_io.h, and io.wren.
- Remove the flags that disable it.
- Remove the overloads for print() with different arity. (It was an
  experiment, but I don't think it's that useful.)
- Remove IO.read(). That will reappear using libuv in the CLI at some
  point.
- Remove IO.time. Doesn't seem to have been used.
- Update all of the tests, docs, etc.

I'm sorry for all the breakage this causes, but I think "System" is a
better name for this class (it makes it natural to add things like
"System.gc()") and frees up "IO" for referring to the CLI's IO module.

test/core/fiber/abort.wren

@@ -2,6 +2,6 @@ var fiber = Fiber.new {
   Fiber.abort("Error message.")
 }
 
-IO.print(fiber.try()) // expect: Error message.
-IO.print(fiber.isDone) // expect: true
-IO.print(fiber.error) // expect: Error message.
+System.print(fiber.try()) // expect: Error message.
+System.print(fiber.isDone) // expect: true
+System.print(fiber.error) // expect: Error message.

test/core/fiber/call.wren

@@ -1,7 +1,7 @@
 var fiber = Fiber.new {
-  IO.print("fiber")
+  System.print("fiber")
 }
 
-IO.print("before") // expect: before
+System.print("before") // expect: before
 fiber.call()       // expect: fiber
-IO.print("after")  // expect: after
+System.print("after")  // expect: after

test/core/fiber/call_done.wren

@@ -1,5 +1,5 @@
 var fiber = Fiber.new {
-  IO.print("call")
+  System.print("call")
 }
 
 fiber.call() // expect: call

test/core/fiber/call_error.wren

@@ -1,6 +1,6 @@
 var fiber = Fiber.new {
   Fiber.abort("Error!")
-  IO.print("should not get here")
+  System.print("should not get here")
 }
 
 fiber.try()

test/core/fiber/call_return_implicit_null.wren

@@ -1,5 +1,5 @@
 var fiber = Fiber.new {
-  IO.print("fiber") // expect: fiber
+  System.print("fiber") // expect: fiber
 }
 
-IO.print(fiber.call()) // expect: null
+System.print(fiber.call()) // expect: null

test/core/fiber/call_return_value.wren

@@ -1,6 +1,6 @@
 var fiber = Fiber.new {
-  IO.print("fiber")
+  System.print("fiber")
   return "result" // expect: fiber
 }
 
-IO.print(fiber.call()) // expect: result
+System.print(fiber.call()) // expect: result

test/core/fiber/call_transferred.wren

@@ -1,12 +1,12 @@
 var main = Fiber.current
 
 var fiber = Fiber.new {
-  IO.print("transferred")
-  IO.print(main.transfer())
-  IO.print("called")
+  System.print("transferred")
+  System.print(main.transfer())
+  System.print("called")
 }
 
 fiber.transfer() // expect: transferred
-IO.print("main") // expect: main
+System.print("main") // expect: main
 fiber.call()     // expect: null
                  // expect: called

test/core/fiber/call_with_value.wren

@@ -1,9 +1,9 @@
 var fiber = Fiber.new {
-  IO.print("fiber")
+  System.print("fiber")
 }
 
 // The first value passed to the fiber is ignored, since there's no yield call
 // to return it.
-IO.print("before")    // expect: before
+System.print("before")    // expect: before
 fiber.call("ignored") // expect: fiber
-IO.print("after")     // expect: after
+System.print("after")     // expect: after

test/core/fiber/call_with_value_done.wren

@@ -1,5 +1,5 @@
 var fiber = Fiber.new {
-  IO.print("call")
+  System.print("call")
 }
 
 fiber.call(1) // expect: call

test/core/fiber/call_with_value_error.wren

@@ -1,6 +1,6 @@
 var fiber = Fiber.new {
   Fiber.abort("Error!")
-  IO.print("should not get here")
+  System.print("should not get here")
 }
 
 fiber.try()

test/core/fiber/call_with_value_transferred.wren

@@ -1,12 +1,12 @@
 var main = Fiber.current
 
 var fiber = Fiber.new {
-  IO.print("transferred")
-  IO.print(main.transfer())
-  IO.print("called")
+  System.print("transferred")
+  System.print(main.transfer())
+  System.print("called")
 }
 
 fiber.transfer()    // expect: transferred
-IO.print("main")    // expect: main
+System.print("main")    // expect: main
 fiber.call("value") // expect: value
                     // expect: called

test/core/fiber/error.wren

@@ -2,6 +2,6 @@ var fiber = Fiber.new {
   "s".unknown
 }
 
-IO.print(fiber.error) // expect: null
-IO.print(fiber.try()) // expect: String does not implement 'unknown'.
-IO.print(fiber.error) // expect: String does not implement 'unknown'.
+System.print(fiber.error) // expect: null
+System.print(fiber.try()) // expect: String does not implement 'unknown'.
+System.print(fiber.error) // expect: String does not implement 'unknown'.

test/core/fiber/is_done.wren

@@ -1,11 +1,11 @@
 var fiber = Fiber.new {
-  IO.print("1")
+  System.print("1")
   Fiber.yield()
-  IO.print("2")
+  System.print("2")
 }
 
-IO.print(fiber.isDone)  // expect: false
+System.print(fiber.isDone)  // expect: false
 fiber.call()            // expect: 1
-IO.print(fiber.isDone)  // expect: false
+System.print(fiber.isDone)  // expect: false
 fiber.call()            // expect: 2
-IO.print(fiber.isDone)  // expect: true
+System.print(fiber.isDone)  // expect: true

test/core/fiber/is_done_after_error.wren

@@ -3,4 +3,4 @@ var fiber = Fiber.new {
 }
 
 fiber.try()
-IO.print(fiber.isDone) // expect: true
+System.print(fiber.isDone) // expect: true

test/core/fiber/resume_caller.wren

@@ -1,16 +1,16 @@
 var b = Fiber.new {
-  IO.print("fiber b")
+  System.print("fiber b")
 }
 
 var a = Fiber.new {
-  IO.print("begin fiber a")
+  System.print("begin fiber a")
   b.call()
-  IO.print("end fiber a")
+  System.print("end fiber a")
 }
 
-IO.print("begin main")
+System.print("begin main")
 a.call()
-IO.print("end main")
+System.print("end main")
 
 // expect: begin main
 // expect: begin fiber a

test/core/fiber/transfer.wren

@@ -1,20 +1,20 @@
 var a = Fiber.new {
-  IO.print("a")
+  System.print("a")
 }
 
 var b = Fiber.new {
-  IO.print("b before")
+  System.print("b before")
   a.transfer()
-  IO.print("b after")
+  System.print("b after")
 }
 
 var c = Fiber.new {
-  IO.print("c before")
+  System.print("c before")
   b.transfer()
-  IO.print("c after")
+  System.print("c after")
 }
 
-IO.print("start") // expect: start
+System.print("start") // expect: start
 
 c.transfer()
 // expect: c before

test/core/fiber/transfer_direct_reenter.wren

@@ -1,9 +1,9 @@
 var F = Fiber.new {
-  IO.print(1)            // expect: 1
-  IO.print(F.transfer()) // expect: null
-  IO.print(2)            // expect: 2
+  System.print(1)            // expect: 1
+  System.print(F.transfer()) // expect: null
+  System.print(2)            // expect: 2
 }
 
 F.call()
 // F remembers its original caller so transfers back to main.
-IO.print(3) // expect: 3
+System.print(3) // expect: 3

test/core/fiber/transfer_indirect_reenter.wren

@@ -1,13 +1,13 @@
 var A = Fiber.new {
-  IO.print(2)
+  System.print(2)
   B.transfer()
-  IO.print("nope")
+  System.print("nope")
 }
 
 var B = Fiber.new {
-  IO.print(1)
+  System.print(1)
   A.transfer()
-  IO.print(3)
+  System.print(3)
 }
 
 B.call()
@@ -15,4 +15,4 @@ B.call()
 // expect: 2
 // expect: 3
 // B remembers its original caller so returns to main.
-IO.print(4) // expect: 4
+System.print(4) // expect: 4

test/core/fiber/transfer_return_call_value.wren

@@ -1,23 +1,23 @@
 var main = Fiber.current
 
 var fiber = Fiber.new {
-  IO.print("fiber 1")
-  IO.print(main.transfer())
+  System.print("fiber 1")
+  System.print(main.transfer())
 
   // Yield to bounce back to main and clear the caller so we don't get a
   // double call() error below.
   Fiber.yield()
 
-  IO.print(main.transfer())
+  System.print(main.transfer())
 }
 
 fiber.transfer() // expect: fiber 1
-IO.print("main 1") // expect: main 1
+System.print("main 1") // expect: main 1
 fiber.call("call 1") // expect: call 1
 
-IO.print("main 2") // expect: main 2
+System.print("main 2") // expect: main 2
 // Transfer back into the fiber so it has a NULL caller.
 fiber.transfer()
 
 fiber.call() // expect: null
-IO.print("main 3") // expect: main 3
+System.print("main 3") // expect: main 3

test/core/fiber/transfer_return_transfer_value.wren

@@ -1,13 +1,13 @@
 var main = Fiber.current
 
 var fiber = Fiber.new {
-  IO.print("fiber")
-  IO.print(main.transfer())
+  System.print("fiber")
+  System.print(main.transfer())
 }
 
 fiber.transfer() // expect: fiber
-IO.print("main") // expect: main
+System.print("main") // expect: main
 fiber.transfer("transfer") // expect: transfer
 
 // This does not get run since we exit when the transferred fiber completes.
-IO.print("nope")
+System.print("nope")

test/core/fiber/transfer_to_done.wren

@@ -1,5 +1,5 @@
 var a = Fiber.new {
-  IO.print("run")
+  System.print("run")
 }
 
 a.call() // expect: run

test/core/fiber/transfer_to_error.wren

@@ -1,6 +1,6 @@
 var a = Fiber.new {
   Fiber.abort("Error!")
-  IO.print("should not get here")
+  System.print("should not get here")
 }
 
 a.try()

test/core/fiber/transfer_to_yielded.wren

@@ -1,7 +1,7 @@
 var fiber = Fiber.new {
-  IO.print("called")
-  IO.print(Fiber.yield())
-  IO.print("transferred")
+  System.print("called")
+  System.print(Fiber.yield())
+  System.print("transferred")
 }
 
 fiber.call()     // expect: called

test/core/fiber/transfer_with_value.wren

@@ -1,20 +1,20 @@
 var a = Fiber.new {
-  IO.print("a")
+  System.print("a")
 }
 
 var b = Fiber.new {
-  IO.print("b before")
+  System.print("b before")
   a.transfer("ignored")
-  IO.print("b after")
+  System.print("b after")
 }
 
 var c = Fiber.new {
-  IO.print("c before")
+  System.print("c before")
   b.transfer("ignored")
-  IO.print("c after")
+  System.print("c after")
 }
 
-IO.print("start") // expect: start
+System.print("start") // expect: start
 
 c.transfer("ignored")
 // expect: c before

test/core/fiber/transfer_with_value_direct_reenter.wren

@@ -1,8 +1,8 @@
 var F = Fiber.new {
-  IO.print(1) // expect: 1
-  IO.print(F.transfer("value")) // expect: value
-  IO.print(2) // expect: 2
+  System.print(1) // expect: 1
+  System.print(F.transfer("value")) // expect: value
+  System.print(2) // expect: 2
 }
 
 F.call()
-IO.print(3) // expect: 3
+System.print(3) // expect: 3

test/core/fiber/transfer_with_value_indirect_reenter.wren

@@ -1,17 +1,17 @@
 var A = Fiber.new {
-  IO.print(2)
+  System.print(2)
   B.transfer("ignored")
-  IO.print("nope")
+  System.print("nope")
 }
 
 var B = Fiber.new {
-  IO.print(1)
+  System.print(1)
   A.transfer("ignored")
-  IO.print(3)
+  System.print(3)
 }
 
 B.call()
 // expect: 1
 // expect: 2
 // expect: 3
-IO.print(4) // expect: 4
+System.print(4) // expect: 4

test/core/fiber/transfer_with_value_to_done.wren

@@ -1,5 +1,5 @@
 var a = Fiber.new {
-  IO.print("run")
+  System.print("run")
 }
 
 a.call() // expect: run

test/core/fiber/transfer_with_value_to_error.wren

@@ -1,6 +1,6 @@
 var a = Fiber.new {
   Fiber.abort("Error!")
-  IO.print("should not get here")
+  System.print("should not get here")
 }
 
 a.try()

test/core/fiber/transfer_with_value_to_yielded.wren

@@ -1,7 +1,7 @@
 var fiber = Fiber.new {
-  IO.print("called")
-  IO.print(Fiber.yield())
-  IO.print("transferred")
+  System.print("called")
+  System.print(Fiber.yield())
+  System.print("transferred")
 }
 
 fiber.call()            // expect: called

test/core/fiber/try.wren

@@ -1,10 +1,10 @@
 var fiber = Fiber.new {
-  IO.print("before")
+  System.print("before")
   true.unknownMethod
-  IO.print("after")
+  System.print("after")
 }
 
-IO.print(fiber.try())
+System.print(fiber.try())
 // expect: before
 // expect: Bool does not implement 'unknownMethod'.
-IO.print("after try") // expect: after try
+System.print("after try") // expect: after try

test/core/fiber/try_when_done.wren

@@ -1,5 +1,5 @@
 var fiber = Fiber.new {
-  IO.print("try")
+  System.print("try")
 }
 
 fiber.try() // expect: try

test/core/fiber/try_without_error.wren

@@ -1,8 +1,8 @@
 var fiber = Fiber.new {
-  IO.print("fiber")
+  System.print("fiber")
 }
 
-IO.print("before")    // expect: before
-IO.print(fiber.try()) // expect: fiber
+System.print("before")    // expect: before
+System.print(fiber.try()) // expect: fiber
                       // expect: null
-IO.print("after")     // expect: after
+System.print("after")     // expect: after

test/core/fiber/type.wren

@@ -1,5 +1,5 @@
 var fiber = Fiber.new {}
-IO.print(fiber is Fiber)      // expect: true
-IO.print(fiber is Object)     // expect: true
-IO.print(fiber is Bool)       // expect: false
-IO.print(fiber.type == Fiber) // expect: true
+System.print(fiber is Fiber)      // expect: true
+System.print(fiber is Object)     // expect: true
+System.print(fiber is Bool)       // expect: false
+System.print(fiber.type == Fiber) // expect: true

test/core/fiber/yield.wren

@@ -1,14 +1,14 @@
 var fiber = Fiber.new {
-  IO.print("fiber 1")
+  System.print("fiber 1")
   Fiber.yield()
-  IO.print("fiber 2")
+  System.print("fiber 2")
   Fiber.yield()
-  IO.print("fiber 3")
+  System.print("fiber 3")
 }
 
 fiber.call()          // expect: fiber 1
-IO.print("main 1")    // expect: main 1
+System.print("main 1")    // expect: main 1
 fiber.call()          // expect: fiber 2
-IO.print("main 2")    // expect: main 2
+System.print("main 2")    // expect: main 2
 fiber.call()          // expect: fiber 3
-IO.print("main 3")    // expect: main 3
+System.print("main 3")    // expect: main 3

test/core/fiber/yield_from_main.wren

@@ -1,3 +1,3 @@
-IO.print("before") // expect: before
+System.print("before") // expect: before
 Fiber.yield()
-IO.print("not reached")
+System.print("not reached")

test/core/fiber/yield_return_call_value.wren

@@ -1,12 +1,12 @@
 var fiber = Fiber.new {
-  IO.print("fiber 1")
-  IO.print(Fiber.yield())
-  IO.print(Fiber.yield())
+  System.print("fiber 1")
+  System.print(Fiber.yield())
+  System.print(Fiber.yield())
 }
 
 fiber.call() // expect: fiber 1
-IO.print("main 1") // expect: main 1
+System.print("main 1") // expect: main 1
 fiber.call("call 1") // expect: call 1
-IO.print("main 2") // expect: main 2
+System.print("main 2") // expect: main 2
 fiber.call() // expect: null
-IO.print("main 3") // expect: main 3
+System.print("main 3") // expect: main 3

test/core/fiber/yield_return_transfer_value.wren

@@ -1,13 +1,13 @@
 var main = Fiber.current
 
 var a = Fiber.new {
-  IO.print("a")
-  IO.print(Fiber.yield())
+  System.print("a")
+  System.print(Fiber.yield())
 }
 
 var b = Fiber.new {
-  IO.print("b")
-  IO.print(Fiber.yield())
+  System.print("b")
+  System.print(Fiber.yield())
 
   a.call()
   a.transfer("value")

test/core/fiber/yield_with_no_caller.wren

@@ -1,7 +1,7 @@
 var a = Fiber.new {
-  IO.print("before") // expect: before
+  System.print("before") // expect: before
   Fiber.yield()
-  IO.print("not reached")
+  System.print("not reached")
 }
 
 // Transfer through a chain of fibers. Since none of them are called, they all
@@ -9,4 +9,4 @@ var a = Fiber.new {
 var b = Fiber.new { a.transfer() }
 var c = Fiber.new { b.transfer() }
 c.transfer()
-IO.print("not reached")
+System.print("not reached")

test/core/fiber/yield_with_value.wren

@@ -1,14 +1,14 @@
 var fiber = Fiber.new {
-  IO.print("fiber 1")
+  System.print("fiber 1")
   Fiber.yield("yield 1")
-  IO.print("fiber 2")
+  System.print("fiber 2")
   Fiber.yield("yield 2")
-  IO.print("fiber 3")
+  System.print("fiber 3")
 }
 
-IO.print(fiber.call())  // expect: fiber 1
+System.print(fiber.call())  // expect: fiber 1
                         // expect: yield 1
-IO.print(fiber.call())  // expect: fiber 2
+System.print(fiber.call())  // expect: fiber 2
                         // expect: yield 2
-IO.print(fiber.call())  // expect: fiber 3
+System.print(fiber.call())  // expect: fiber 3
                         // expect: null

test/core/fiber/yield_with_value_from_main.wren

@@ -1,3 +1,3 @@
-IO.print("before") // expect: before
+System.print("before") // expect: before
 Fiber.yield(1)
-IO.print("not reached")
+System.print("not reached")

test/core/fiber/yield_with_value_with_no_caller.wren

@@ -1,7 +1,7 @@
 var a = Fiber.new {
-  IO.print("before") // expect: before
+  System.print("before") // expect: before
   Fiber.yield(1)
-  IO.print("not reached")
+  System.print("not reached")
 }
 
 // Transfer through a chain of fibers. Since none of them are called, they all
@@ -9,4 +9,4 @@ var a = Fiber.new {
 var b = Fiber.new { a.transfer() }
 var c = Fiber.new { b.transfer() }
 c.transfer()
-IO.print("not reached")
+System.print("not reached")