Add Merkle proofs to the protocol

This MR adds Merkle proofs to the protocol. This exposes 4 new functions: {produce,verify}_{proof,stream} to produce and verify Merkle proofs with the current Tezos context.

This is a follow-up from !4138 (merged) which was introducing proofs in the shell.

A follow-up MR will address #2362 (closed).