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/use_global_in_method.wren

@@ -3,11 +3,11 @@ var global = "global"
 
 class Foo {
   method {
-    io.write(global)
+    IO.write(global)
   }
 
   static classMethod {
-    io.write(global)
+    IO.write(global)
   }
 }