"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/method/newlines.wren

@@ -7,14 +7,14 @@ class Foo {
 var foo = Foo.new()
 
 // Allow newlines after commas and before ")".
-IO.print(foo.method("a",
+System.print(foo.method("a",
 
     "b"
 
     )) // expect: method a b
 
 // Allow newlines after commas and before "]".
-IO.print(foo["a",
+System.print(foo["a",
 
     "b"