Expose the nn_send & nn_recv feature that are not accessible through the reader API

[blabaere]

For nn_send, we could start by just exposing a non-blocking version of write. I quite don't see how to integrate nn_allocmsg correctly for the moment.
For nn_recv, there are four cases and two of them are already available in the reader API. We need to add the non-blocking versions of read and read_to_end.

[thehydroimpulse]

Agreed!

[blabaere]

Now only remains to be implemented nn_send with the nn_msg zero-copy option. I'd like a new function to expose the fact that the buffer must not be used after being sent. It's the kind of constraint that rust should be able to express quite nicely.