History Stat Comparison

Adjust LMR by comparing history stats
with opponent (prior ply).

STC:
LLR: 2.96 (-2.94,2.94) [0.00,5.00]
Total: 27754 W: 5066 L: 4824 D: 17864

LTC:
LLR: 2.95 (-2.94,2.94) [0.00,5.00]
Total: 216596 W: 28157 L: 27343 D: 161096

Bench: 5437729

src/search.h

@@ -43,6 +43,7 @@ struct Stack {
   Move excludedMove;
   Move killers[2];
   Value staticEval;
+  Value history;
   bool skipEarlyPruning;
   int moveCount;
   CounterMoveStats* counterMoves;