async-std has been discontinued; use smol instead

https://docs.rs/async-std/latest/async_std/
This will obviously result in breaking changes to the api.

I was working on replacing async-std with smol.
I have gotten it to build and I have ~3 flaky tests that sometimes work and sometimes not.

I noticed that you have a branch for tokio support. Should i rebase my changes for that branch instead?