Replace deprecated async-std with smol #90
Description
async-std has been deprecated in favor of smol, as suggested in the readme.
I will link two PRs to this issue that approach this in different ways. One adds a new feature similar to how tokio was added, smol-runtime, which is less intrusive of a change but does add some more to ownership of the library. The second approach replaces async-std completely making smol the new default.
This was a bit tedious so it's possible I missed something, but for what it's worth I made sure all checks, tests, and benches still succeeded. Some documentation (like the README and the CHANGELOG) may still require some updates. Feel free to tweak/close either PR as you please.
As a feature: #88
As a replacement: #89