Renamed sort indexes Rank to Score. Before the skiplist, scores and rank
were the same thing (the score of an item was based on its rank in the array).
However, skipslists introduce a distinct concept of Rank and Score. The score
is the absolute value given to the value. It can be anything, it can be
duplicated, there can be gaps. A rank, however, is the actual position
within the structure.

The rank is important when we're doing offsets with a range. A value's rank
is essentially its offset. Since the two are interchangable, we can apply
range + offset in a single pass (get the rank of the range, add that to the
offset)