Merge: model: introduce model_json

This PR makes all model entities Jsonable when `model_json` is imported.

Not sure if I was supposed to use the serialization process or not in this case? @xymus what do you think?

This is mostly hand picked services chosen to provide a comprehensive model representation usable by other tools like nitdoc, nitx, web tools or else.

Pull-Request: #2024
Reviewed-by: Jean Privat <jean@pryen.org>