refactor: convert IO from singleton instance to static class with metaclass native methods

Replace the global `io` singleton object with a static `IO` class, moving the `write` native from the instance to the metaclass. Update all benchmark, example, and test files to use `IO.write()` instead of `io.write()`.

test/closure/reuse_closure_slot.wren

@@ -3,7 +3,7 @@
 
   {
     var a = "a"
-    f = fn io.write(a)
+    f = fn IO.write(a)
   }
 
   {