A waker implementation that keeps count of its clones and panics if any clones were leaked.
This is similar in use cases to the
waker-fn crate, and trades away the alloc dependency,
paying with the possibility of panicking at a leak.
Panicking at termination is particularly not an issue when tasks never return anyway.
This could conceivably be taken out of riot-wrappers into a standalone crate – but then DropGuard needs to be generalized.