"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/constructor/named.wren

@@ -5,10 +5,10 @@ class Foo {
   toString { _field }
 }
 
-IO.print(Foo.named()) // expect: named
-IO.print(Foo.other()) // expect: other
+System.print(Foo.named()) // expect: named
+System.print(Foo.other()) // expect: other
 
 // Returns the new instance.
 var foo = Foo.named()
-IO.print(foo is Foo) // expect: true
-IO.print(foo.toString) // expect: named
+System.print(foo is Foo) // expect: true
+System.print(foo.toString) // expect: named