Feature request: Add FromRawFd to UnixStream

[laptou]

I would like to be able to convert raw file descriptors into UnixStream without having to convert to Async<std::os::unix::net::UnixStream> first.

[notgull]

I don't believe that there's a way to implement FromRawFd on UnixStream without introducing a potential panic. Given that smol-rs/async-net#21 introduces a way to convert an OwnedFd to a UnixStream using TryFrom, and how one can unsafely convert a RawFd to an OwnedFd, this issue should be resolved.