Etherlink: generic storage crate

Context

This is been on my mind for a long time, we had a lot of duplicate generic function/helpers for the storage that was spread amongst our different crate (kernel / indexable_storage / evm_execution).

Each generic function/helper is extracted and documented in the tezos_storage crate.

Whenever we had to write a new storage-related module, there were some redundant function that were re-written. It shouldn't be the case anymore.

Just for more context, I know that Sylvain is currently working on an MR where he had to duplicate some helper code related to storage, this was the final straw that led to this MR.

Manually testing the MR

CI should be enough.

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