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.
"M3" is for "milestone 3" -- my target to fix the technical debt
that I think will be bad if we ship even a pre-alpha with it.
These aren't necessarily _all_ must-resolve, but they're all
must-look-at.
Closes#15
Nick Mathewson