feat(transports/unix-stream): add

[nabijaczleweli]

Description

This allows dialling with and listening on AF_UNIX/SOCK_STREAM sockets.

Ripped 100% off transports/tcp, as in I cp -r tcp unix-streamed it.

Uses proposed encoding from multiformats/multiaddr#174 (comment)

Notes & open questions

I called the empty tag enum UnixStrm due to a name collision. I don't know what the purpose of it is anyway, but.

As for the encoding, I hope to move it entirely into multiaddr; transports/tcp has

Ok(SocketAddr::new(ipv4.into(), port)),

and

fn ip_to_multiaddr(ip: IpAddr, port: u16) -> Multiaddr {
    Multiaddr::empty().with(ip.into()).with(Protocol::Tcp(port))
}

so the same thing should be available once that lands in multiaddr.

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• A changelog entry has been made in the appropriate crates ‒ I don't see a root entry in transports/tcp so I've left it empty