Add `from_bytes` methods, improve serialization.

[afck]

Serializing a public key with bincode wastes 8 bytes:

assert_eq!(pk.to_bytes().len(), 48);
assert_eq!(bincode::serialize(&pk).unwrap().len(), 56);

Can we bring that size down to 48 bytes?
In either case, we should add a from_bytes method that turns a compressed public key back into a PublicKey struct.