"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/object/not.wren

@@ -1,4 +1,4 @@
 class Foo {
   construct new() {}
 }
-IO.print(!Foo.new()) // expect: false
+System.print(!Foo.new()) // expect: false

test/core/object/same.wren

@@ -1,27 +1,27 @@
 // Value types compare by value.
-IO.print(Object.same(true, true)) // expect: true
-IO.print(Object.same(true, false)) // expect: false
+System.print(Object.same(true, true)) // expect: true
+System.print(Object.same(true, false)) // expect: false
 
-IO.print(Object.same(null, null)) // expect: true
+System.print(Object.same(null, null)) // expect: true
 
-IO.print(Object.same(1 + 2, 2 + 1)) // expect: true
-IO.print(Object.same(1 + 2, 2 + 2)) // expect: false
+System.print(Object.same(1 + 2, 2 + 1)) // expect: true
+System.print(Object.same(1 + 2, 2 + 2)) // expect: false
 
-IO.print(Object.same(1..2, 1..2)) // expect: true
-IO.print(Object.same(1..2, 1..3)) // expect: false
+System.print(Object.same(1..2, 1..2)) // expect: true
+System.print(Object.same(1..2, 1..3)) // expect: false
 
-IO.print(Object.same("ab", "a" + "b")) // expect: true
-IO.print(Object.same("ab", "a" + "c")) // expect: false
+System.print(Object.same("ab", "a" + "b")) // expect: true
+System.print(Object.same("ab", "a" + "c")) // expect: false
 
 // Different types are never the same.
-IO.print(Object.same(null, false)) // expect: false
-IO.print(Object.same(true, 2)) // expect: false
-IO.print(Object.same(1..2, 2)) // expect: false
-IO.print(Object.same("1", 1)) // expect: false
+System.print(Object.same(null, false)) // expect: false
+System.print(Object.same(true, 2)) // expect: false
+System.print(Object.same(1..2, 2)) // expect: false
+System.print(Object.same("1", 1)) // expect: false
 
 // Classes compare by identity.
-IO.print(Object.same(Bool, Num)) // expect: false
-IO.print(Object.same(Bool, Bool)) // expect: true
+System.print(Object.same(Bool, Num)) // expect: false
+System.print(Object.same(Bool, Bool)) // expect: true
 
 // Other types compare by identity.
 class Foo {
@@ -29,8 +29,8 @@ class Foo {
 }
 
 var foo = Foo.new()
-IO.print(Object.same(foo, foo)) // expect: true
-IO.print(Object.same(foo, Foo.new())) // expect: false
+System.print(Object.same(foo, foo)) // expect: true
+System.print(Object.same(foo, Foo.new())) // expect: false
 
 // Ignores == operators.
 class Bar {
@@ -39,5 +39,5 @@ class Bar {
 }
 
 var bar = Bar.new()
-IO.print(Object.same(bar, bar)) // expect: true
-IO.print(Object.same(bar, Bar.new())) // expect: false
+System.print(Object.same(bar, bar)) // expect: true
+System.print(Object.same(bar, Bar.new())) // expect: false

test/core/object/to_string.wren

@@ -1,4 +1,4 @@
 class Foo {
   construct new() {}
 }
-IO.print(Foo.new().toString == "instance of Foo") // expect: true
+System.print(Foo.new().toString == "instance of Foo") // expect: true

test/core/object/type.wren

@@ -1,13 +1,13 @@
 class Foo {}
 
 // Object's class is a class.
-IO.print(Object is Class) // expect: true
+System.print(Object is Class) // expect: true
 
 // Its metatype is also a class.
-IO.print(Object.type is Class) // expect: true
+System.print(Object.type is Class) // expect: true
 
 // The metatype's metatype is Class.
-IO.print(Object.type.type == Class) // expect: true
+System.print(Object.type.type == Class) // expect: true
 
 // Object has a distinct metaclass.
-IO.print(Object.type.name) // expect: Object metaclass
+System.print(Object.type.name) // expect: Object metaclass