Trim off finished futures from the Event._waiters

[ZhukovAlexander]

Current implementation of Event.wait uses dequeue.remove(e), wich has O(n) complexity. We can do better by utilizing this time to also remove all finished futures from the queue on the way up until we reach the future that is in a pending state. This will make subsequent calls to Event.wait or Event.set to perform less work on a waiters queue

[sethmlarson]

Pretty sure that fut is always at index 0 within _waiters by the time that it yields. remove() only removes the first encountered element so it'll stop there, so probably no speed-up using this trimming method. Instead of using remove() could just use popleft() as it's probably faster.

[ZhukovAlexander]

@SethMichaelLarson , I think you are right. Though, the side effect of this algorithm is that it will also trim all finished futures that come after the current one, so it still has an effect.

[sethmlarson]

@ZhukovAlexander When would that occur? Each future trims itself as it's finished so there's only ever one finished future in _waiters. The only time I can think where that would happen is if code was muddling with the internals.

[gvanrossum]

If fut always occurs at index 0 there is no O() improvement from using popleft(), so the point of the diff is moot -- let's close this PR then.