Conditionally compile atomic implementations

[fwcd]

This enables compilation e.g. on 32-bit platforms where 64-bit atomics may be unavailable.

[udoprog]

Hi!

Since it's using cfg_target_has_atomic, it makes musli nightly only. I think the only option is to put it behind a feature flag the user is responsible for turning off. Unless there's some other way to make it work on stable.

[fwcd]

I've updated it to use feature flags. Once rust-lang/rust#94039 stabilizes, we could deprecate the features and revert to the original implementation (a96056c).

[fwcd]

Any update on this?

[udoprog]

Looks Ok, I've been working on some other things and haven't come back to this project yet. But I'll rebase this for now.

I might tweak the features before release, but I haven't decided how yet.

[udoprog]

@fwcd So I was looking around how other projects was approaching this, and target_has_atomic is stable since 1.60. So what I did in the last PR was to try and lean on that instead. While the visibility of Atomic* depends on target_has_atomic_load_store (which is nightly only) it seems like there's no reasonable case where the two would diverge.

Could you take the commit I pushed for a spin in your project and see if it works as expected?