Implement different trait for generating and verifying PoP about bls HOT 3 CLOSED

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

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f4d137c would

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

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b6ba31d

• Add extra-message for proof of knowledge.

from bls.