Poseidon Support for k256 Hash To Curve

[shreyas-londhe]

Hi, we have a requirement to use the Poseidon hash function instead of the sha256 hash function in the hash2curve for the curve k256. I'm more than happy to implement it and make a PR with a little bit of help. Please let me know if this would be a good addition and where I can start from.

[tarcieri]

We don't currently have an implementation of Poseidon, although there was some discussion of adding one (and/or Poseidon2) here: RustCrypto/hashes#1

k256 is designed so you can implement algorithms like Poseidon in downstream crates. It should be possible to implement it in such a way that it's generic over elliptic curves as well. As such, it probably shouldn't go in k256 itself.