Type-safe bind methods (especially for TCP and UDP sockets)

[nagisa]

While ∅MQ has its own design, it is tailored for C and requires passing in the socket parameters as a string. We could do better! We have SocketAddr and all sorts of other cool ways to make the API less stringly.

[rotty]

Indeed! What I imagine this would involve:

• Go through the endpoint types zmq_connect manpage, and construct an Endpoint enum with a variant for each supported transport.
• Each transport variant should be able to map all the variants described in the corresponding manpage (e.g. zmq_tcp), while retaining as much structure as possible.
• I'm not yet sure if we want a catch-all Unknown variant which simply contains a &str; I'd rather go without that for now.

The resulting enum should then implement Display for conversion into a string (or buffer), and FromStr for the opposite direction.

The Endpoint enum should be unit-tested properly, at least ensuring proper round-tripping from &str to Endpoint and back to String.

This affects the following Socket methods:

• connect, disconnect, monitor and get_last_endpoint.

I have no idea how to incorporate this sensibly without breaking API (without introducing additional methods, which I don't think is worth it), so this would have to go wait for 0.10. If you, @nagisa, or someone else wants to cook up a PR, go for it -- I don't know when I'd come around to giving it a go myself.