Prerequisites for 1.114.0

This patch reverts to using the rust library vendoring approach, and makes some improvements to that process. I'd like to have a discussion about this in the long term, but at present I don't see value in trying to use the debcargo to track a swarm of dependencies which do not provide any stable API. Moreover, my efforts to get rust packages into debcargo have not resulted in enough motion to actually build synapse against accepted packages (the proposed packages do, however, work).

If this approach is acceptable, I'll push the actual (large) patch with the vendored sources, and debian/copyright.

I'm locally running 1.114.0 without issue (and using this approach).