glommio extensibility

[benjumanji]

I would like to use glommio for a small embedded project. I don't need the speed, but I do like the programming model. Glommio afaict currently lacks anything similar to async-io's Async, i.e. some means to take a random fd and stick on the reactor for reads/writes. Specifically I would like to port rtnetlink to glommio, but I don't think I can do it without effectively adding it to this crate, and I doubt anyone else is particularly interested in that kind of bloat. Obviously I can just run two runtimes, it just seems a bit daft.

Thoughts?