Some API suggestions

[vorner]

Hello

I was reading through the code as it is already written (because I might find a use for it sometime soon) and I'd have a few suggestions, if I may (I wanted to discuss them first, before I start making pull requests).

Basically, it's almost what I need, except that I'd like to be able to both receive and send multipart messages ‒ not only receive them. Would you agree if I provided the code?

Furthermore, I noticed some TODO notes about using a Codec (https://github.com/rotty/zmq-tokio/blob/master/src/lib.rs#L122). I believe Codec is wrong thing to do (because it handles framing and ZMQ does framing already). I think the right thing to do is simply return the Vec's and convert them using .map() on the stream.