Draft: lib_shell: prevalidator: better control of effects

Context

As !3965 (merged) shows, we have been recently bitten by a bug stemming from the fact that some functions of the mempool have too many inputs.

In the spirit of avoiding similar bugs in the future, this MR better controls outputs, so that required updates to mutable state in general is easier to do.

📑 Note: the first commit makes sense on its own. We could take it alone if the rest is deemed irrelevant.

Manually testing the MR

• Observe that this MR doesn't change the behavior

Checklist

• NA Document the interface of any function added or modified (see the coding guidelines)
• NA Document any change to the user interface, including configuration parameters (see node configuration)
• NA Provide automatic testing (see the testing guide).
• NA 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).
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