Integrate merkle Multi-proofs into hash-based PC schemes

[mmagician]

Summary

Integrate Batched Proof for multiple merkle openings: arkworks-rs/crypto-primitives#130.

This should get integrated both into Brakedown and Ligero. The performance will be especially noticeable in Brakedown, which requires to prove ~10x as many merkle paths to achieve the same security.

As a pre-requisite to taking on this task, we should check if the multi-proof can be improved (especially in the parallel scenario). Brakedown should really optimize for prover speed, so switching to a slower version of multi-proof is counterproductive here.

Upon completion, we need to benchmark the schemes end-to-end and check for concrete savings on the 1) prover 2) verifier 3) proof size.