A `contains` function

[FireflyInTheDusk]

Thanks a lot for the lib, it came in clutch while writing pathfinding 😄

It seems to lack a quick way to check if a key is present in the queue or not, besides bool exists = queue.get_priority(key).first.

Let me know if this is something you'd like to add, I added one in my fork and could PR it.

https://github.com/FireflyInTheDusk/updatable_priority_queue/commit/43132867e3169ea11fbdf9ea5a592a5020f39392