[Twisted-Python] reactor for Linux io_uring

[Ian Haywood]

In investigating async file I/O I came across this. In a nutshell it's 
the new epoll()

It's marginally more efficient although this is only apparent at very 
high loads. What's more interesting is that io_uring accepts files as 
well as network/pipe handles: avoiding the need for threads.

Here's a good intro: https://unixism.net/loti/index.html

If people think an IoUringReactor is worthwhile I'll open a ticket and 
make a start.

However it will need a reviewer... :-)

Ian