<queue>: priority_queue::push_range could conditionally push_heap

[CaseyCarter]

See discussion at #2806 (comment).

When inserting a small number of elements k into a large priority_queue, make_heap's complexity of 3N can be greater than consecutive calls to push_heap (k log N). We need to determine what the actual relationship is given our implementation's constant factors, and further determine if we believe the potential perf improvement is worth the cost of additional codesize.