More efficient sponge

Right now, the trait `CryptographicSponge` is only implemented for Poseidon sponges. These are comparatively inefficient for out-circuit use. A PR is open which implements the trait for the more efficient Merlin sponge: [https://github.com/arkworks-rs/crypto-primitives/pull/136](https://github.com/arkworks-rs/crypto-primitives/pull/136). Let's make the swap once that PR is merged and stable.