"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/inheritance/do_not_inherit_static_methods.wren

@@ -1,5 +1,5 @@
 class Foo {
-  static methodOnFoo { IO.print("foo") }
+  static methodOnFoo { System.print("foo") }
 }
 
 class Bar is Foo {}

test/language/inheritance/inherit_fields.wren

@@ -7,8 +7,8 @@ class Foo {
   }
 
   fooPrint {
-    IO.print(_field1)
-    IO.print(_field2)
+    System.print(_field1)
+    System.print(_field2)
   }
 }
 
@@ -21,8 +21,8 @@ class Bar is Foo {
   }
 
   barPrint {
-    IO.print(_field1)
-    IO.print(_field2)
+    System.print(_field1)
+    System.print(_field2)
   }
 }
 

test/language/inheritance/inherit_from_closure.wren

@@ -2,7 +2,7 @@ var ClosureType
 
 {
   var a = "a"
-  ClosureType = Fn.new { IO.print(a) }.type
+  ClosureType = Fn.new { System.print(a) }.type
 }
 
 class Subclass is ClosureType {} // expect runtime error: Class 'Subclass' cannot inherit from built-in class 'Fn'.

test/language/inheritance/inherit_methods.wren

@@ -1,16 +1,16 @@
 class Foo {
-  methodOnFoo { IO.print("foo") }
-  method(a) { IO.print("foo") }
-  method(a, b, c) { IO.print("foo") }
-  override { IO.print("foo") }
+  methodOnFoo { System.print("foo") }
+  method(a) { System.print("foo") }
+  method(a, b, c) { System.print("foo") }
+  override { System.print("foo") }
 }
 
 class Bar is Foo {
   construct new() {}
-  methodOnBar { IO.print("bar") }
-  method(a, b) { IO.print("bar") }
-  method(a, b, c, d) { IO.print("bar") }
-  override { IO.print("bar") }
+  methodOnBar { System.print("bar") }
+  method(a, b) { System.print("bar") }
+  method(a, b, c, d) { System.print("bar") }
+  override { System.print("bar") }
 }
 
 var bar = Bar.new()

test/language/inheritance/inherited_fields_in_closure.wren

@@ -26,11 +26,11 @@ class Bar is Foo {
 }
 
 var bar = Bar.new()
-IO.print(bar.closeOverFooGet.call().call()) // expect: Foo field
-IO.print(bar.closeOverBarGet.call().call()) // expect: Bar field
+System.print(bar.closeOverFooGet.call().call()) // expect: Foo field
+System.print(bar.closeOverBarGet.call().call()) // expect: Bar field
 bar.closeOverFooSet.call().call()
-IO.print(bar.closeOverFooGet.call().call()) // expect: new foo value
-IO.print(bar.closeOverBarGet.call().call()) // expect: Bar field
+System.print(bar.closeOverFooGet.call().call()) // expect: new foo value
+System.print(bar.closeOverBarGet.call().call()) // expect: Bar field
 bar.closeOverBarSet.call().call()
-IO.print(bar.closeOverFooGet.call().call()) // expect: new foo value
-IO.print(bar.closeOverBarGet.call().call()) // expect: new bar value
+System.print(bar.closeOverFooGet.call().call()) // expect: new foo value
+System.print(bar.closeOverBarGet.call().call()) // expect: new bar value

test/language/inheritance/is.wren

@@ -11,12 +11,12 @@ var a = A.new()
 var b = B.new()
 var c = C.new()
 
-IO.print(a is A) // expect: true
-IO.print(a is B) // expect: false
-IO.print(a is C) // expect: false
-IO.print(b is A) // expect: true
-IO.print(b is B) // expect: true
-IO.print(b is C) // expect: false
-IO.print(c is A) // expect: true
-IO.print(c is B) // expect: true
-IO.print(c is C) // expect: true
+System.print(a is A) // expect: true
+System.print(a is B) // expect: false
+System.print(a is C) // expect: false
+System.print(b is A) // expect: true
+System.print(b is B) // expect: true
+System.print(b is C) // expect: false
+System.print(c is A) // expect: true
+System.print(c is B) // expect: true
+System.print(c is C) // expect: true