⚡ Avoid opposite side offset calculation

[eduherminio]

• Replace opposite side offset calculation. taking advantage that these two are equivalent:

  `(int) whitePieceIndex + oppositeSideOffset`;
  `(int) blackPieceIndex - offset`;

• Simplify WasProducedByAValidMove since it doesn't need to check for king existence

8+0.08 (WIP)

Score of Lynx 1236 - nooffiset vs Lynx 1223 - main: 6711 - 6482 - 4287  [0.507] 17480
...      Lynx 1236 - nooffiset playing White: 4006 - 2587 - 2147  [0.581] 8740
...      Lynx 1236 - nooffiset playing Black: 2705 - 3895 - 2140  [0.432] 8740
...      White vs Black: 7901 - 5292 - 4287  [0.575] 17480
Elo difference: 4.6 +/- 4.5, LOS: 97.7 %, DrawRatio: 24.5 %
SPRT: llr 1.97 (66.8%), lbound -2.94, ubound 2.94