Merge branch 'kk/prio-queue-cascade-sift' into seen

prio_queue_get() has been optimized by using a cascade-down approach
(promoting the smaller child at each level and sifting up the last
element from the leaf vacancy), which halves the number of comparisons
per extract-min operation in the common case.

* kk/prio-queue-cascade-sift:
  prio-queue: use cascade-down for faster extract-min