Expose Keccak-256 hashing in Compact

[andreyjv]

[andreyjv]

https://github.com/shieldedtech/product/blob/PRD-Keccak256/PRD-keccak256.md

[shieldedmark]

Once JubJub point (#85) and ‘secp256k1 point’ (#104) completes

[kmillikin]

I don't think this needs any elliptic curves as a dependency? It does need support in the on-chain runtime (I don't know if that's exposed yet), and we'll need to add it as a new ZKIR operation unless we can decompose it into existing ones.