Add flexible Receiver enum and remove Object::default_receiver()

+ Receiver enum has been added to replace the Option<ObjectRef> wherein None
  meant default_receiver(). This allows us to set NativeReceivers on things
  without creating new nuketypes to do so. It will be especially useful for
  implementing `system`.

- Object::default_receiver() has been removed in favor of object constructors
  setting the receiver to a NativeReceiver on creation.