Scoru: split [Sc_rollup_storage]

Context

Extract from Sc_rollup_storage

• Sc_rollup_commitment_storage_repr
• Sc_rollup_inbox_storage_repr
• Sc_rollup_stake_storage_repr
• Sc_rollup_refutation_storage_repr

Manually testing the MR

Semantics is not changed, the code is just moved around, the CI should prevail.

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