convert: improve docstrings, comments.

changelog: turn {de,en}code_extra methods into functions The methods were not really methods -- they didn't use 'self'. Having them as functions in the module it useful for other modules (like the commitsigs extension) that want to recompute the changeset hash and thus want to encode dicts the same way as changelog does it. Removed the underbars from their names at the same time.

merge with crew-stable