Remove the dependency on async-std

As discussed here, removing the async-std dependency and keeping async-io will make this crate usable on ESP-IDF as well.

And might broaden up a bit the target audience, as folks who are into the smol echosystem, but not really willing to depend on all of async-std, might give it a chance.

We might also need to replace nix with something else (socket2/rustix/libc), as it is unclear if nix will build for ESP IDF.