Add pawn cache

[sroelants]

Not super stoked on the implementation. Feels like there should be ways to simplify this further. Also not sure how well this is going to play together with #276.

Elo   | 18.57 +- 8.21 (95%)
SPRT  | 8.0+0.08s Threads=1 Hash=16MB
LLR   | 2.95 (-2.94, 2.94) [0.00, 5.00]
Games | N: 2584 W: 731 L: 593 D: 1260
Penta | [33, 274, 554, 384, 47]
https://chess.samroelants.com/test/273/

bench 7142602