poseidon circuit implementation

[redshiftzero]

This ticket is for adding support for computing hashes in circuit, probably via an extension trait on PlonkCircuit. A pre-req is #55 (which may end up changing the Poseidon parameters and/or implementation we're using). For proofs so far, we need:

• Rate 1: for computing note commitments and asset generators
• Rate 3: for nullifier derivation
• Rate 4: TCT proofs of inclusion

We won't need rate 2 because we won't have a binary NCT anymore.

Related upstream tickets:

• Poseidon hash support (optimized / non-optimized) EspressoSystems/jellyfish#29
• Poiseidon Hash Implementation ZK-Garage/plonk#104

[redshiftzero]

For a first pass at this I'm thinking of using PoseidonSpongeVar internally from ark-sponge. We have implemented in our poseidon-parameters crate conversion of our parameters struct to the struct ark-sponge expects so it should be straightforward to use