Use Poseidon sponge in Halo2 transcript in prover and out-of-circuit verifier

[Deleted user]

The Halo2 proving system needs to be modified to use the Poseidon sponge to create the proof transcript instead of Blake2 or another non-circuit friendly hash.

This requires:

• Finishing #6
• Creating a ChallengeScalar type to replace the byte Challenge255
• Creating a PoseidonTranscript which uses the Poseidon sponge and ChallengeScalar
• Handle prefixes in a better way than absorbing prefix twice (related to manual padding)
• Cleanup, e.g. merging Transcript and PoseidonTranscript to reduce redundant code
• Verify correct behavior with good tests
• Merge to some upstream