[Search] Sort moves one-by-one instead of in bulk

[dannyhammer]

Presently, moves are sorted in the search function with sort_by_cached_key(). This was fine initially, but will be a source of slowdowns in the future.

Because of how branches are pruned in the a/b search, we are not necessarily searching all available moves. We are likely (and ideally) only searching a small fraction. So, if we are not searching all moves, why are we sorting all of them?

Rustic has a good page about this in it's book. You can use a modified version of selection sort to do this.