SCORU: Add Merkelized inbox

Context

This MR introduces the merkelization scheme for smart contract rollups inboxes, i.e., it defines how the current level messages are represented as Merkle trees and how the history of an inbox is represented as a Merkle skip list.

Fix #2553 (closed) #2308 (closed)

Manually testing the MR

dune exec src/proto_alpha/lib_protocol/test/unit/main.exe -- test "^\[Unit\] sc rollup inbox$"
dune exec src/proto_alpha/lib_protocol/test/unit/main.exe -- test "^\[Unit\] skip list$"

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Provide automatic testing (see the testing guide).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR