Tx_rollup: Introduce L2 batches with compact encoding

Context

The goal of the draft is to provide the concrete definitions for what can be stored in a L2 batch, along with compact encoding to be sure they do not wast too much byte.

The reasoning is detailed in tx_rollup_l2_batch.mli: the smaller the layer-2 transactions are, the more we can put layer-2 transactions in a Tezos block.

Manually testing the MR

TODO

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, the Development Version section of CHANGES.md for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR