replace code-gen macros in tcp and uds with async-io

[dvc94ch]

While working on refactoring the transport trait to accomodate quic or port reuse in tcp, it seemed like the code needs some maintainance. I think async-io should work fine for any executor, so the TokioTcp and Tcp stuff can be simplified to just Tcp. Unless there are any objections I'll submit a PR as soon as smol-rs/async-io#36 gets fixed.

[ericjmiller]

I think this could affect implementation of #1820.

[dvc94ch]

This issue was resolved I guess.