⚡ Add a Board array to Position to track where pieces are indexed by square

[eduherminio]

Not the expected speedup:

Test  | perf/board-bitboard
Elo   | 0.25 +- 1.70 (95%)
SPRT  | 8.0+0.08s Threads=1 Hash=32MB
LLR   | -2.26 (-2.25, 2.89) [0.00, 3.00]
Games | 79402: +24850 -24793 =29759
Penta | [2687, 8751, 16815, 8714, 2734]
https://openbench.lynx-chess.com/test/444/

Update SPRT

Test  | perf/board-bitboard
Elo   | 1.05 +- 2.41 (95%)
SPRT  | 8.0+0.08s Threads=1 Hash=32MB
LLR   | 2.90 (-2.25, 2.89) [-3.00, 1.00]
Games | 33570: +9440 -9339 =14791
Penta | [768, 3906, 7382, 3915, 814]
https://openbench.lynx-chess.com/test/728/

Merging since this will help some HCE heuristics such as making PieceAttackedByPawnPenalty easily indexable by piece