Draft: Move away from async-std

Still in Draft status as I've removed the async-std dependemncy, yet nix is still around, so the code will likely not build on ESP IDF.

Next step is to either replace nix with socket2, or upstream changes to nix which make it compatible with ESP IDF. The latter would be very annoying though, as that would be the n-th "safe Posix APIs" crate I need to address...