bubbleUp() {
    let index = this.size - 1;
    const parent = (i) => Math.ceil(i / 2) - 1;
    while (parent(index) >= 0 && this.comparator(parent(index), index) > 0) {
      this.swap(parent(index), index);
      index = parent(index);
    }
  }
Move new element upwards on the heap, if it's out of order
@runtime O(log n)