Merge: astbuilder: Introduction of new construction services

Add two new services in the astbuilder module:
- Add a way in ModelBuilder to create AST and module entity at same time (`create_method`, `create_class`, `create_attribute`).

- Add a new property in MEntity `create_ast_representation` to return a ast representation of `self`.

Pull-Request: #2815
Reviewed-by: Jean Privat <jean@pryen.org>