Tokio support

[lilianmoraru]

Plans to switch from mio to tokio? This would make scaproust play nice with the async ecosystem(also compiling the code throws a lot of deprecation warnings).

[blabaere]

I've been scratching my head about this for quite some time now (see the TODO.MD file) and I think this really needs to be done, for a lot of reasons. The problem is:

• I don't see yet how to implement load-balancing and fair queuing with tokio the way they are in nanomsg. That's because mio readiness events are not exposed via tokio.
• I almost don't have time at the moment for side projects and switching to tokio is a big change.

When I'll have some time, I will probably create a new library over tokio rather than changing scaproust. It will probably be closer to mango and less like the original C API.