"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/implicit_receiver/locals_shadow_getter.wren

@@ -4,15 +4,15 @@ class Foo {
   bar { "getter" }
 
   test {
-    IO.print(bar) // expect: getter
+    System.print(bar) // expect: getter
 
     {
-      IO.print(bar) // expect: getter
+      System.print(bar) // expect: getter
       var bar = "local"
-      IO.print(bar) // expect: local
+      System.print(bar) // expect: local
     }
 
-    IO.print(bar) // expect: getter
+    System.print(bar) // expect: getter
   }
 }