"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/module/change_imported_value/change_imported_value.wren

@@ -1,11 +1,11 @@
 import "module" for Module, Other
 
-IO.print(Module) // expect: before
+System.print(Module) // expect: before
 
 // Reassigning the variable in the other module does not affect this one's
 // binding.
 Other.change
-IO.print(Module) // expect: before
+System.print(Module) // expect: before
 
 // But it does change there.
 Other.show // expect: after

test/language/module/change_imported_value/module.wren

@@ -7,6 +7,6 @@ class Other {
   }
 
   static show {
-    IO.print(Module)
+    System.print(Module)
   }
 }