oneshot has potential Use After Free when used asynchronously

Summary

There is a race condition that can lead to a use-after-free if a `oneshot::Receiver` is polled but then dropped instead of polled to completion. This could happen if the receiver future was cancelled while receiving, for example by being wrapped in a timeout future or similar. When the `Receiver` is polled (`Future::poll`) it writes a waker to the channel and sets it to the `RECEIVING` state. If the `Receiver` was then dropped (instead of polled to completion), the `Drop` implementation on `Receiver` unconditionally swapped the channel state to `DISCONNECTED` and only after doing so it read back its waker from the heap allocation and dropped it. The problem is that the `DISCONNECTED` state could be observed by the `Sender`, which would lead to it deallocating the channel heap memory. If the `Sender` manage to free the channel before the `Receiver` managed to proceed to dropping the waker, then the `Receiver` would read from the freed channel memory (Use After Free). The fix was submitted in https://github.com/faern/oneshot/pull/74 and published as part of `oneshot` version `0.1.12`.