Extract general::SocketAddr and its unix friends to their own crate.
This is mostly code motion; you will likely want to review at least some of these commits with git show --color-moved.
The rationale for doing this is that I'd like to use general::SocketAddr in RPC client code, which doesn't want anything to do with rtcompat or the broader extended asynchroverse.
Calling this a Blocker since until it is merged (