Swap atomic-polyfill for portable-atomic

[thejpster]

https://crates.io/crates/atomic-polyfill says:

⚠️ THIS CRATE IS DEPRECATED ⚠️

Use portable-atomic instead. It supports many more architectures and more efficient ways of emulating atomics.

[thejpster]

Oh, there's a PR in #328

[newAM]

Fixed in #328 🎉