Use int instead of Value for history related stats.

history related scores are not related to evaluation based scores.
For example, can easily exceed the range -VALUE_INFINITE,VALUE_INFINITE.
As such the current type is confusing, and a plain int is a better match.

tested for no regression:

STC:
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 43693 W: 7909 L: 7827 D: 27957

No functional change.

Closes #1070

src/movegen.h

@@ -38,7 +38,7 @@ enum GenType {
 
 struct ExtMove {
   Move move;
-  Value value;
+  int value;
 
   operator Move() const { return move; }
   void operator=(Move m) { move = m; }