Consider using fixed gaps instead of a halfway mark when appending

[dmarkow]

Even with an upper limit of 2.1 billion, 32 items added to a list sequentially hits the max possible rank. This means that after just 32 items, the entire list has to be rebalanced. Right now, every insertion after this shifts the entire list. Even after #12 is fixed, it will still have to rebalance every ~25 inserts.

If we made a fixed gap of 2,000 for example, this would allow 1 million appended items before hitting the max rank, while still leaving enough of a gap to allow for around a dozen inserts between existing items until rebalancing would be needed.