EVM/Mockup: collect block propositions in kernel

Context

Fixes #4879 (closed).

This MR contains a (really) simple and straightforward representation of the inbox messages interpretation and how to collect them. It also contain a dummy blueprint's queue mechanism without any safety check.

Methods can be added gradually to our needs.

You will notice a few code comments in inbox.rs, they're here to attest the emptiness of this shell of collect phase and how they are supposed to be filled according to Thomas' document.

Everything related to the delayed inbox is omitted for now.

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, 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