Allow usage on no_std targets and targets using portable-atomic

[notgull]

This pull requests replaces this crate's dependency on libstd with libcore, allowing it to be used on no_std targets. In addition, it adds a feature called atomic-polyfill, which uses the crate of the same name to provide AtomicUsize on no_std targets.

[taiki-e]

The atomic-polyfill and the feature-based approach it recommends can very easily introduce unsoundness when the dependency enables that feature. (Even if they no longer provide a default implementation, even if they warn about it in their documentation. -- I have seen many libraries in the async ecosystem where the similar feature to select TLS or runtime is enabled by default or always enabled. So, I believe that such an approach is fundamentally fragile.)

See also mvdnes/spin-rs#114 and tokio-rs/bytes#461 (comment).

[notgull]

I see that in the issues you linked portable-atomic as a potential alternative. Would you be fine with that instead?

[taiki-e]

Yeah, I'm fine with that.

[notgull]

I've ported this code to use portable-atomic instead.

[notgull]

I just added a check to the CI that builds on a no_std target.

[taiki-e]

Thanks!