Make Piece an integer

[eduherminio]

027eced

Elo   | 1.11 +- 4.09 (95%)
SPRT  | 8.0+0.08s Threads=1 Hash=32MB
LLR   | 2.89 (-2.25, 2.89) [-5.00, 0.00]
Games | N: 17488 W: 5551 L: 5495 D: 6442
Penta | [689, 2023, 3299, 2009, 724]
https://openbench.lynx-chess.com/test/84/

e0d2b80 vs previous

[WIP]
Elo   | -10.86 +- 11.58 (95%)
SPRT  | 8.0+0.08s Threads=1 Hash=32MB
LLR   | -1.13 (-2.25, 2.89) [-5.00, 0.00]
Games | N: 2208 W: 670 L: 739 D: 799
Penta | [105, 240, 450, 237, 72]
https://openbench.lynx-chess.com/test/85/

0e4bd95:

Elo   | 5.74 +- 6.47 (95%)
SPRT  | 8.0+0.08s Threads=1 Hash=32MB
LLR   | 2.96 (-2.25, 2.89) [-5.00, 0.00]
Games | N: 6902 W: 2209 L: 2095 D: 2598
Penta | [258, 742, 1353, 824, 274]
https://openbench.lynx-chess.com/test/87/