function push(compare, heap, item) {
  heap.push(item);
  siftDown(compare, heap, 0, heap.length - 1);
}
Function used to push an item into a heap represented by a raw array.

@param {function} compare - Comparison function.
@param {array}    heap    - Array storing the heap's data.
@param {any}      item    - Item to push.