Make `USED_PARALLELISM` atomic

[nathaniel-daniel]

Fixes #1491

This fixes UB with the USED_PARALLELISM variable, rationale described in #1491. Additionally, static mut is difficult to use correctly, to the point where it is likely to be removed in a future edition. Instead, I have replaced the bool with an AtomicBool, which can be used without unsafe.

This approach is superior to #1492, as using a mutex for a bool is wasteful.

[Narsil]

Thanks a lot for this !

Much better indeed than the existing code.

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[Narsil]

Let's ignore the clippy lints, I'll tackle them in another PR.