FD handling: avoid unnecessary dynamic downcasts

[RalfJung]

The eventfd and socket implementations had to work with a FileDescriptionRef and then dynamically downcast it to the right type. That should be unnecessary, we should be able to just track what the actual type of these references is.

So that's what this PR implements: we now have FileDescriptionRef<T> where T indicates the type of the FD. We also have DynFileDescriptionRef for when the underlying type is not statically known. What makes this tricky is that we want to also have an FdId available via the reference. We can use CoercePointee to make Rc<(FdId, T)> a dyn-compatible pointer type that can be the receiver of read/write/close. However, that's still not quite enough since we also want dynamic downcasting, and that is only possible with Rc<dyn Any>, so the usual "make Any a supertrait" does not work... we need FdIdWith<Self>: Any, which doesn't work with supertrait upcasting so it needs a custom little helper trait.

This also fixes the annoying wart that FileDescription::{read,write} received two self pointers: a &self and a FileDescriptionRef.

The 2nd commit switches epoll_wait over to a strong ref, which is now a lot easier. That fixes #4065.