"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/language/module/change_imported_value/change_imported_value.wren

@@ -1,11 +1,11 @@
 import "module" for Module, Other
 
-IO.print(Module) // expect: before
+System.print(Module) // expect: before
 
 // Reassigning the variable in the other module does not affect this one's
 // binding.
 Other.change
-IO.print(Module) // expect: before
+System.print(Module) // expect: before
 
 // But it does change there.
 Other.show // expect: after

test/language/module/change_imported_value/module.wren

@@ -7,6 +7,6 @@ class Other {
   }
 
   static show {
-    IO.print(Module)
+    System.print(Module)
   }
 }

test/language/module/cyclic_import/a.wren

@@ -1,9 +1,9 @@
 // nontest
-IO.print("start a")
+System.print("start a")
 
 var A = "a value"
-IO.print("a defined ", A)
+System.print("a defined " + A)
 import "b" for B
-IO.print("a imported ", B)
+System.print("a imported " + B)
 
-IO.print("end a")
+System.print("end a")

test/language/module/cyclic_import/b.wren

@@ -1,9 +1,9 @@
 // nontest
-IO.print("start b")
+System.print("start b")
 
 var B = "b value"
-IO.print("b defined ", B)
+System.print("b defined " + B)
 import "a" for A
-IO.print("b imported ", A)
+System.print("b imported " + A)
 
-IO.print("end b")
+System.print("end b")

test/language/module/implicitly_imports_core/module.wren

@@ -1,15 +1,15 @@
 // nontest
-IO.print(Bool)
-IO.print(Class)
-IO.print(Fiber)
-IO.print(Fn)
-IO.print(List)
-IO.print(Map)
-IO.print(MapKeySequence)
-IO.print(MapValueSequence)
-IO.print(Null)
-IO.print(Num)
-IO.print(Object)
-IO.print(Range)
-IO.print(Sequence)
-IO.print(String)
+System.print(Bool)
+System.print(Class)
+System.print(Fiber)
+System.print(Fn)
+System.print(List)
+System.print(Map)
+System.print(MapKeySequence)
+System.print(MapValueSequence)
+System.print(Null)
+System.print(Num)
+System.print(Object)
+System.print(Range)
+System.print(Sequence)
+System.print(String)

test/language/module/inside_block/inside_block.wren

@@ -4,7 +4,7 @@ if (true) {
   import "module" for Module
   // expect: ran module
 
-  IO.print(Module) // expect: from module
+  System.print(Module) // expect: from module
 }
 
-IO.print(Module) // expect: outer
+System.print(Module) // expect: outer

test/language/module/inside_block/module.wren

@@ -1,3 +1,3 @@
 // nontest
 var Module = "from module"
-IO.print("ran module")
+System.print("ran module")

test/language/module/module_dir/module_dir.wren

@@ -1,3 +1,3 @@
 import "something" for Index
 
-IO.print(Index) // expect: index
+System.print(Index) // expect: index

test/language/module/multiple_variables/module.wren

@@ -5,4 +5,4 @@ var Module3 = "from module three"
 var Module4 = "from module four"
 var Module5 = "from module five"
 
-IO.print("ran module")
+System.print("ran module")

test/language/module/multiple_variables/multiple_variables.wren

@@ -3,8 +3,8 @@ import "module" for Module1, Module2, Module3, Module4, Module5
 // Only execute module body once:
 // expect: ran module
 
-IO.print(Module1) // expect: from module one
-IO.print(Module2) // expect: from module two
-IO.print(Module3) // expect: from module three
-IO.print(Module4) // expect: from module four
-IO.print(Module5) // expect: from module five
+System.print(Module1) // expect: from module one
+System.print(Module2) // expect: from module two
+System.print(Module3) // expect: from module three
+System.print(Module4) // expect: from module four
+System.print(Module5) // expect: from module five

test/language/module/no_variable/module.wren

@@ -1,2 +1,2 @@
 // nontest
-IO.print("ran module")
+System.print("ran module")

test/language/module/shared_import/a.wren

@@ -1,5 +1,5 @@
 // nontest
-IO.print("a")
+System.print("a")
 import "shared" for Shared
 var A = "a " + Shared
-IO.print("a done")
+System.print("a done")

test/language/module/shared_import/b.wren

@@ -1,5 +1,5 @@
 // nontest
-IO.print("b")
+System.print("b")
 import "shared" for Shared
 var B = "b " + Shared
-IO.print("b done")
+System.print("b done")

test/language/module/shared_import/shared.wren

@@ -1,3 +1,3 @@
 // nontest
-IO.print("shared")
+System.print("shared")
 var Shared = "shared"

test/language/module/shared_import/shared_import.wren

@@ -8,5 +8,5 @@ import "b" for B
 // expect: b
 // expect: b done
 
-IO.print(A) // expect: a shared
-IO.print(B) // expect: b shared
+System.print(A) // expect: a shared
+System.print(B) // expect: b shared

test/language/module/simple_import/module.wren

@@ -1,3 +1,3 @@
 // nontest
 var Module = "from module"
-IO.print("ran module")
+System.print("ran module")

test/language/module/simple_import/simple_import.wren

@@ -1,4 +1,4 @@
 import "module" for Module
 // expect: ran module
 
-IO.print(Module) // expect: from module
+System.print(Module) // expect: from module

test/language/module/unknown_variable/module.wren

@@ -1,3 +1,3 @@
 // nontest
 var Module = "from module"
-IO.print("ran module")
+System.print("ran module")