Make sure that signing certs are signed by the right identity key,
to prevent a recurrence of #16530. Also make sure that the master
identity key we find on disk matches the one we have in RAM, if we
have one.
This is for #16581.
For prop220, we have a new ed25519 certificate type. This patch
implements the code to create, parse, and validate those, along with
code for routers to maintain their own sets of certificates and
keys. (Some parts of master identity key encryption are done, but
the implementation of that isn't finished)
