Now, a Relay is always valid.  This required some changes to the
API: all_relays() has to return a new UncheckedRelay type that might
or might not be valid, and the functions on Relay and ChanTarget
that return ed25519 identities need to return an Ed25519Identity,
not an ed25519::PublicKey.

This change required some new encoding/decoding/conversion functions
on Ed25519Identity.