PriorityQueue: use O(logN) algorithm for dequeue!(pq, key)
Still return a tuple from `peek(pq)`
Fix type-stability of setindex!(::PriorityQueue, val, key) (return val)
Merge pull request #8011 from JuliaLang/teh/pq

PriorityQueue: use O(logN) algorithm for dequeue!(pq, key)