Issue a warning when coroutine is never awaited

[wyfo]

Following #3588, coroutines should issue a warning when they are never awaited, to mimic Python coroutine behavior.

However, the best place to put this warning seems to be in __del__, so it would requires #2479 before.

"Never awaited" means (for Python coroutines) that the coroutine has not been polled at least once. Regarding current coroutine implementation, it means that waker field is still None when __del__ is executed.

[davidhewitt]

For what it's worth, last time I investigated __del__ it seemed tricky to support for all interpreter versions, abi3 etc.

I imagine we could get something working for at least a subset of versions, and that'd also be completely acceptable for this sort of warning. If we gave timely feedback to upstream we could probably ensure perfect support for 3.13+ too.

[wyfo]

I feel a little bit stupid, but this warning can simply be put in a Drop implementation 😅