unexpand: refactor to accept buffer as a parameter

[xtqqczze]

Closes #11787

[oech3]

Does it share a stack at one uumain call?
fn unexpand is not called in loop.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/tail-n0f (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/expand/bounded-memory is now passing!

[xtqqczze]

It's just a draft to see CodSpeed results

[oech3]

You can accel CodSpeed by removing other *yml and non-unexpand section.

[oech3]

https://codspeed.io/uutils/coreutils/branches/xtqqczze%3AGH11787?utm_source=github&utm_medium=check&utm_content=button&q=unexpand

It seems vec is needed even vec! is ugly.