namespaces: introduce a generic way to map between names and nodes This patch begins the work to provide a way to register a namespace to handle 'names'. Benefits of this would be, - improved templating: This would provide {name} which could output any branch, bookmark, tag, or any extension registered namespace all without having the extension doing any extra work - improved tab completion: Since this provides a single source of all 'names', tab completion would not need to know of each namespace - changeset lookup: Similar to before, a unified place to get all 'names' will allow finding changesets without any extension code having to reimplement this Also, d226fe36e362 has shown us that for internal code which expects a certain type of method or behavior, we should provide an easy way for extensions to check this behavior.