Implement different trait for generating and verifying PoP

[drskalman]

Sample interface is on the branch: origin/skalman-pok-new-interface

• Generate trait should be implemented by the secret key.
• Verify trait should be implemented by the public key.

[burdges]

I'm not sure we need a trait, either we need a second public key type, or else the sign_pok and verify_pok methods do serialization and deserialization.

We had a trait before largely because I made a mistake and thought we could avoid proofs-of-knowldge using Dan Boneh's trick on the full signer set, meaning we'd process the whole PKI together, which indicated a trait, but.. If there were 2^40 adversarial signers then one could break this approach using https://people.eecs.berkeley.edu/~daw/papers/genbday.html and now if the adversarial signer set has merely 256 signers then you can break this using https://eprint.iacr.org/2020/945.pdf

[drskalman]

f4d137c would

• [X] rebase the trait divided branch to verify brach.
• [X] Divide the traits.
• [X] implement the secret key for first trait.
• [X] implement the public key for the second trait.
• [X] test that verification works.

[drskalman]

b6ba31d

• [X] Add extra-message for proof of knowledge.