Switch to Merkle_list for withdrawal

Context

Use the same kind of Merkle_list for withdrawal as we use for inboxes. That way we don't have three different flavors of Merkle proof (this one, the old one, and the one used for rejections)

Manually testing the MR

Tests, hopefully, pass. No user-visible changes.

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