Support acquire_many (and variants) in Semaphore

[afranchuk]

It'd be great if Semaphore supported _many variants for acquisition. To guarantee fairness, Event would need to change (as you'd need to check whether there are enough permits for the head of the listener list and only notify if that were the case). If interested, I have a set of changes for event-listener which I think does the trick (adding a new CondEvent type which allows data to be associated with listeners and a predicate over that data to be passed with notify). I also have another set of changes for async-lock which allow multiple permits to be acquired (I haven't fully tested them together, but will do so if this sounds like a good idea to you). Thanks!