* remove hard-coded ef_construction_multiplier
* instead, let ef_construction go up and down automatically as needed
* as needed means that expanding the queue changes the result much
* much is defined by the queue stiffness, as in Hooke's law
* also search_layer() now returns only as many elements as needed, the
  caller no longer needs to overallocate result arrays for throwaway nodes
* change _downheap() to return the position where the element ended up