implemented endgame tablebases!

essentially, chess is solved with 7 pieces or fewer. so instead of trying to calculate the perfect move in a complicated position, we can just look it up in a precomputed database of all positions with 7 pieces or fewer. this is a huge boost for endgame play, and also lets the engine play perfectly in those positions instead of relying on heuristics. the syzygy tablebases are the standard in the chess programming community, so i used them!