"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/inherited_fields_in_closure.wren

@@ -26,11 +26,11 @@ class Bar is Foo {
 }
 
 var bar = Bar.new()
-IO.print(bar.closeOverFooGet.call().call()) // expect: Foo field
-IO.print(bar.closeOverBarGet.call().call()) // expect: Bar field
+System.print(bar.closeOverFooGet.call().call()) // expect: Foo field
+System.print(bar.closeOverBarGet.call().call()) // expect: Bar field
 bar.closeOverFooSet.call().call()
-IO.print(bar.closeOverFooGet.call().call()) // expect: new foo value
-IO.print(bar.closeOverBarGet.call().call()) // expect: Bar field
+System.print(bar.closeOverFooGet.call().call()) // expect: new foo value
+System.print(bar.closeOverBarGet.call().call()) // expect: Bar field
 bar.closeOverBarSet.call().call()
-IO.print(bar.closeOverFooGet.call().call()) // expect: new foo value
-IO.print(bar.closeOverBarGet.call().call()) // expect: new bar value
+System.print(bar.closeOverFooGet.call().call()) // expect: new foo value
+System.print(bar.closeOverBarGet.call().call()) // expect: new bar value