"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/class/equality.wren

@@ -1,13 +1,13 @@
-IO.print(Num == Num)  // expect: true
-IO.print(Num == Bool) // expect: false
+System.print(Num == Num)  // expect: true
+System.print(Num == Bool) // expect: false
 
 // Not equal to other types.
-IO.print(Num == 123)  // expect: false
-IO.print(Num == true) // expect: false
+System.print(Num == 123)  // expect: false
+System.print(Num == true) // expect: false
 
-IO.print(Num != Num)  // expect: false
-IO.print(Num != Bool) // expect: true
+System.print(Num != Num)  // expect: false
+System.print(Num != Bool) // expect: true
 
 // Not equal to other types.
-IO.print(Num != 123)  // expect: true
-IO.print(Num != true) // expect: true
+System.print(Num != 123)  // expect: true
+System.print(Num != true) // expect: true

test/core/class/name.wren

@@ -1,13 +1,13 @@
 class Foo {}
 
-IO.print(Foo.name) // expect: Foo
-IO.print(Foo.type.name) // expect: Foo metaclass
+System.print(Foo.name) // expect: Foo
+System.print(Foo.type.name) // expect: Foo metaclass
 
 // Make sure the built-in classes have proper names too.
-IO.print(Object.name) // expect: Object
-IO.print(Bool.name) // expect: Bool
-IO.print(Class.name) // expect: Class
+System.print(Object.name) // expect: Object
+System.print(Bool.name) // expect: Bool
+System.print(Class.name) // expect: Class
 
 // And metaclass names.
-IO.print(Object.type.name) // expect: Object metaclass
-IO.print(Bool.type.name) // expect: Bool metaclass
+System.print(Object.type.name) // expect: Object metaclass
+System.print(Bool.type.name) // expect: Bool metaclass

test/core/class/supertype.wren

@@ -5,11 +5,11 @@ class Bar is Foo {}
 class Baz is Bar {}
 
 // A class with no explicit superclass inherits Object.
-IO.print(Foo.supertype == Object) // expect: true
+System.print(Foo.supertype == Object) // expect: true
 
 // Otherwise, it's the superclass.
-IO.print(Bar.supertype == Foo) // expect: true
-IO.print(Baz.supertype == Bar) // expect: true
+System.print(Bar.supertype == Foo) // expect: true
+System.print(Baz.supertype == Bar) // expect: true
 
 // Object has no supertype.
-IO.print(Object.supertype) // expect: null
+System.print(Object.supertype) // expect: null

test/core/class/type.wren

@@ -1,15 +1,15 @@
 class Foo {}
 
 // A class is a class.
-IO.print(Foo is Class) // expect: true
+System.print(Foo is Class) // expect: true
 
 // Its metatype is also a class.
-IO.print(Foo.type is Class) // expect: true
+System.print(Foo.type is Class) // expect: true
 
 // The metatype's metatype is Class.
-IO.print(Foo.type.type == Class) // expect: true
+System.print(Foo.type.type == Class) // expect: true
 
 // And Class's metatype circles back onto itself.
-IO.print(Foo.type.type.type == Class) // expect: true
-IO.print(Foo.type.type.type.type == Class) // expect: true
-IO.print(Foo.type.type.type.type.type == Class) // expect: true
+System.print(Foo.type.type.type == Class) // expect: true
+System.print(Foo.type.type.type.type == Class) // expect: true
+System.print(Foo.type.type.type.type.type == Class) // expect: true