counter move heuristic
Idea mentionned by Don Dailey on talkchess, recently proven successful by Joona in Stockfish. Apparently this is a very old idea, coming from the days where memory ressources were scarce and hash tables were therefore not so practical.

There are many variations of the idea to try, but let's start simple:
- indexing by color/piece/tsq so as to be consistent with history heuristic.
- use the counter move to sort it as a 3-rd killer.

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counter move heuristic: v2
this time use fsq instead of piece
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Revert "counter move heuristic: v2"

This reverts commit 211a293c1a4761308595647bd861511d405fcad6.

signature = 7276221