Improve implementation of `merge`

[gjcairo]

merge currently allocates more than it should on next calls, by creating new arrays with no pre-reserved capacity. What’s more, in its current form of allowing max 3 streams to be merged, this array could actually just be a tuple and avoid the allocation altogether.

There are comments throughout the implementation that suggest the idea was to eventually use variadic generics to merge N streams. We should decide if this is something we actually want to do, and either reserve enough capacity in the continuations array to avoid the constant resizing, or stick to a tuple if we won’t allow more than 3 (or some other fixed number of) streams.

[bartoszirzyk]

That would also be a great improvement for combineLatest to be able to accept N sequences