"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/for/close_over_loop_variable.wren

@@ -1,7 +1,7 @@
 var list = []
 
 for (i in [1, 2, 3]) {
-  list.add(Fn.new { IO.print(i) })
+  list.add(Fn.new { System.print(i) })
 }
 
 for (f in list) f.call()

test/language/for/closure_in_body.wren

@@ -2,7 +2,7 @@ var list = []
 
 for (i in [1, 2, 3]) {
   var j = i + 1
-  list.add(Fn.new { IO.print(j) })
+  list.add(Fn.new { System.print(j) })
 }
 
 for (f in list) f.call()

test/language/for/newline_after_for.wren

@@ -1,2 +1,2 @@
 for // expect error
-(i in [1, 2, 3]) IO.print(i)
+(i in [1, 2, 3]) System.print(i)

test/language/for/newline_before_in.wren

@@ -1,2 +1,2 @@
 for (i // expect error
-  in [1]) IO.print(i)
+  in [1]) System.print(i)

test/language/for/only_evaluate_sequence_once.wren

@@ -1,9 +1,9 @@
 var f = Fn.new {
-  IO.print("evaluate sequence")
+  System.print("evaluate sequence")
   return [1, 2, 3]
 }
 
-for (i in f.call()) IO.print(i)
+for (i in f.call()) System.print(i)
 // expect: evaluate sequence
 // expect: 1
 // expect: 2

test/language/for/return_closure.wren

@@ -1,6 +1,6 @@
 var f = Fn.new {
   for (i in [1, 2, 3]) {
-    return Fn.new { IO.print(i) }
+    return Fn.new { System.print(i) }
   }
 }
 

test/language/for/return_inside.wren

@@ -4,5 +4,5 @@ var f = Fn.new {
   }
 }
 
-IO.print(f.call())
+System.print(f.call())
 // expect: 1

test/language/for/syntax.wren

@@ -1,14 +1,14 @@
 // Single-expression body.
-for (i in [1]) IO.print(i)
+for (i in [1]) System.print(i)
 // expect: 1
 
 // Block body.
 for (i in [1]) {
-  IO.print(i)
+  System.print(i)
 }
 // expect: 1
 
 // Newline after "in".
 for (i in
-  [1]) IO.print(i)
+  [1]) System.print(i)
 // expect: 1

test/language/for/truth.wren

@@ -6,27 +6,27 @@ class Iter {
 
 // False and null are false.
 for (n in Iter.new(false)) {
-  IO.print("bad")
+  System.print("bad")
   break
 }
 
 for (n in Iter.new(null)) {
-  IO.print("bad")
+  System.print("bad")
   break
 }
 
 // Everything else is true.
 for (n in Iter.new(true)) {
-  IO.print("true") // expect: true
+  System.print("true") // expect: true
   break
 }
 
 for (n in Iter.new(0)) {
-  IO.print(0) // expect: 0
+  System.print(0) // expect: 0
   break
 }
 
 for (n in Iter.new("")) {
-  IO.print("string") // expect: string
+  System.print("string") // expect: string
   break
 }