Creates a priority queue. Currently backed by binary heap. The priorities are {term(), number()} tuples, where the 2nd element defines the priority on the 1st element. Options: :comparator - the boolean function that compares 2 priorities (default is Kernel.</2) :keep_lesser - insert/2,3 will ignore new priority for the key if it's not strictly less (as decided by comparator) than the current one (default is true)