tokio-util: expose key used in DelayQueue's Expired

[tobz]

Motivation

It's useful to be able to extract the Key assigned to a given entry in a DelayQueue when being handed back an expired item, so that callers can interact with the delay queue in a correct fashion.

Solution

We've exposed the Key used in Expired<T> such that it can be not only accessed from Expired<T>, but is now also Copy, Eq, and Hash, so it can be easily used in hash maps and sets, as well as trivially copied, since it's just a usize under the hood.

Signed-off-by: Toby Lawrence toby@nuclearfurnace.com