Pre/post `&mut` argument: remove `future` in favor of `old`

[W95Psp]

• make the proc macro parse both ::hax_lib::_final and _final
• detect if (locally) there is any name clash, and ask the user to use ::hax_lib::_final instead

We chatted about that, and we'll adopt the style of Verus / Prusti / Rustc proposal: use old to refer to the old value, and otherwise that's the future value.

[github-actions]

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[W95Psp]

Still something we want, we need to schedule it before too much code is written in that style.

[github-actions]

This issue has been marked as stale due to a lack of activity for 60 days. If you believe this issue is still relevant, please provide an update or comment to keep it open. Otherwise, it will be closed in 7 days.

[W95Psp]

I'll bump priority for that one, it's still important to get that done soon.