impl Stream for DelayQueue

[rubdos]

Motivation

8a7e577 deleted impl Stream for DelayQueue. It also said

Since the Stream implementation is optional, types that are logically
streams provide async fn next_* functions to obtain the next value.
Avoiding the next() name prevents fn conflicts with StreamExt::next().

However, DelayQueue only provided poll_expired, which makes DelayQueue almost unusable in async contexts.

Solution

This PR implements Stream on DelayQueue behind the feature flag stream, which is only a few characters away from the origin implementation that existed before 8a7e577.

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If a test is useful for this contribution, or any changes are needed, please let me know. I'm unsure what kind of test to implement for this, so a pointer to an example test for another module would be appreciated in that case.

[carllerche]

Thanks for doing this 👍