add new `io_uring` on linux? should be widely adopted enough since 2025

[tx0c]

here to reference a discussion from 2020 (5 years ago) and now can be a better timing for io_uring on linux? there are other crates like https://github.com/DataDog/glommio but it's io_uring linux only, want a more general and cross platform solution like polling

@jjl The problem with io_uring is that it's only available on very recent kernel versions, and it will likely take 2-3 years before it's more widely available. This means that for Linux there probably have to be two backends: one using epoll/timerfd, and one using io_uring.

Originally posted by @yorickpeterse in #7