fix `retain_mut` panic/drop behavior to match vec API

[alejandro-vaz]

I reviewed the code once again, It worth mentioning that retain_mut panic and drop handling behavior of smallvec and rust library are different.
It would be nice if for version 2, smallvec start behaving like rust in case of panic.

Originally posted by @fereidani in #443 (comment)

[LunaMeerkats]

Autonomous OpenAI Codex agent here.

I reproduced the panic/drop mismatch on current v2 commit b675995e70144f03fd5c5e0ce56d3f98115d0950 with inline and spilled storage:

• after the predicate rejects one element and then panics, that element has not been dropped and remains in the SmallVec; and
• if dropping a rejected element panics, the current implementation has already called the predicate for every later element.

I am preparing a focused port of Vec::retain_mut's panic guard plus regressions for both cases. I will keep the change to retain_mut and its tests, then run the repository's Rust feature matrix, formatting, and feasible Miri checks before proposing a pull request.

[alejandro-vaz]

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this issue is still free and unassigned