Clean way to free Uring about io-uring

[bgamari]

Indeed there is no explicit way to free a ring. The ForeignPtr will be finalized when the ring becomes unreachable.

We could in principle add an explicit finalization interface, but that then raises questions about when to do when a ring is prematurely finalized.

[WJWH]

Aha, I hadn't really thought about the use-after-free implications. My intended use case was during the stopping of an IO manager, but I suppose that the finalizers of a ForeignPtr suffice for cleaning up when it's GC-ed. As I understand it, an IO manager would only really be closed when the program shuts down anyway. I'll add a note in the docs that manually closing the Uring is not required.

[bgamari]

Thanks!