Implement RefUnwindSafe for Atomic

[sarsko]

Atomics should be RefUnwindSafe, and currently they are not. This PR makes them RefUnwindSafe.

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[bkragl]

Two things that can be done separately.

First, it seems like Mutex and RwLock explicitly implement UnwindSafe, although that shouldn't be necessary.

Second, I think it would be nice to add tests that assert the auto traits that we expect our sync primitives to implement. Something like https://github.com/indexmap-rs/indexmap/blob/184fe4bfcba20e21242c924220170b98be157d45/src/map/core.rs#L636-L642.

[sarsko]

First, it seems like Mutex and RwLock explicitly implement UnwindSafe, although that shouldn't be necessary.

True, though doing it explicitly is a low-cost way to avoid an accidental footgun (if their internals are ever changed). It also makes their documentation match that of their std counterpart, through moving UnwindSafe up from the "Auto Trait Implementations" section.

Second, I think it would be nice to add tests that assert the auto traits that we expect our sync primitives to implement. Something like indexmap-rs/indexmap@184fe4b/src/map/core.rs#L636-L642.

Yes, agreed.