ENH: io_uring instrumentation support

[shanedsnyder]

The io_uring interface for asynchronous I/O (https://man.archlinux.org/man/io_uring.7.en) is gaining popularity, but we don't have any support for instrumenting it in Darshan, yet.

We should investigate whether Darshan can properly instrument this API, and additionally ensure it doesn't impose any overheads that would impact it's performance. The key trick for properly instrumenting is in being able to map completion queue events with submission queue events, which will likely require that Darshan hijack user pointers and implement it's own tracking.