BinaryHeap: Use full sift down in .pop()

BinaryHeap: Use full sift down in .pop()

.sift_down can either choose to compare the element on the way down (and
place it during descent), or to sift down an element fully, then sift
back up to place it.

A previous PR changed .sift_down() to the former behavior, which is much
faster for relatively small heaps and for elements that are cheap to
compare.

A benchmarking run suggested that BinaryHeap::pop() suffers
improportionally from this, and that it should use the second strategy
instead. It's logical since .pop() brings last element from the
heapified vector into index 0, it's very likely that this element will
end up at the bottom again.