Use a pre-sorted index for the score table

Created by: arteymix

It's possible to do a full Gale-Shapley by considering indices of descending probabilities from the score table and reversing them into the corresponding duplex.

That'll mean that we will have both a score table and an index table. The same contiguous data structure would be kept, with the difference that we would instead traverse the pair of (target site, mirna) in descending probability.