Also changes the way the internal data is organized
to match the module comment of io_uring.

… hieararchy.

Instead of a separate entity, UringEventLoop now derives from EpollEventLoop. There is still a lot of duplication in terms of FD slot management and the submit() call that was only done once per process() is now potentially done multiple times.

Fixes compile errors on older compiler versions when not explicitly using the uring driver.