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/assign_to_closure.wren

@@ -4,15 +4,15 @@ var g = null
 {
   var local = "local"
   f = fn {
-    io.write(local)
+    IO.write(local)
     local = "after f"
-    io.write(local)
+    IO.write(local)
   }
 
   g = fn {
-    io.write(local)
+    IO.write(local)
     local = "after g"
-    io.write(local)
+    IO.write(local)
   }
 }