Introduce #model and .model_name to Messages.
Introduce #model and .model_name to Messages.

Details:
* Mainly because Aequitas' i18n message transformer looks for these
  methods when creating a localized error message.
* But thinking it would be nice to have the introspection to
  match up the kind and model_name that implements the kind.
* #model is essentially an alias to the class.

byu/serf