Proto: add Merkle_list module for TORU

Context

The TORU project requires a Merkle tree structure to which new elements can be added, as opposed to S.MERKLE_TREE which only computes the root of a Merkle tree given a list of elements.

This MR introduces an incremental Merkle accumulator in lib_protocol named Merkle_list (avoiding clashes with either S.MERKLE_TREEor the incremental Merkle tree in lib_sapling) which exposes the interface of a Merkleized list.

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR