Refactor tor-linkspec once more.

This is a successor to !646 (closed) and !659 (closed): It refactors out the traits related to having keys, and this time it adds a new set of functionality for making relay identity more abstract so as to avoid locking us into this particular set of keys forever.

I've marked at as Draft because it should not be merged as-is: I split up one of the larger commits into a few logical squash! commits to make it easier to review. We should squash those before we merge.

This is also not a complete reworking of all our code that deals with identities: there are TODO comments left in a number of places to explain what will still need to change as we move ahead.

Closes #428 (closed).