Proto/AI: move Slashed_deposits table outside of Contract subcontext

What

Replace Storage.Contract.Slashed_deposits, which was under the Contract subcontext, with an independant table Storage.Slashed_deposits.

Why

This makes it possible to iterate over this table without needing to iterate over all delegates (or even worse, over all contracts, which was the natural way to do it with the former structure).

How

Storage.Contract.Slashed_deposits was a new table in P (because the one that existed in O had already been renamed to Storage.Contract.Slashed_deposits__Oxford), so there is no need to do anything about the migration.

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