"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/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()