io_uring support

[caelunshun]

This is fairly low priority, but lately there's been lots of discussion about the new io_uring interface in kernels >= 5.1. I imagine we could make good use of this for chunk IO and networking.

The only (relatively) safe io_uring crate I know of rio, but it's licensed under GPL. We'll have to wait for alternatives to pop up.

[Schuwi]

https://github.com/withoutboats/ringbahn might be something to keep an eye on, but it is not really usable yet