Convert killers to a vector

Add infrastructure to threat killer moves as a vector,
this will allow us to easily parametrize the number of
killer moves, instead of hardcode this value to two as is now.

This patch just add the infrastructure, no functional
change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/search.h

@@ -41,6 +41,7 @@
 
 const int PLY_MAX = 100;
 const int PLY_MAX_PLUS_2 = 102;
+const int KILLER_MAX = 4;
 
 
 ////
@@ -56,8 +57,9 @@ struct SearchStack {
   Move pv[PLY_MAX];
   Move currentMove;
   Value currentMoveCaptureValue;
-  Move mateKiller, killer1, killer2;
+  Move mateKiller;
   Move threatMove;
+  Move killers[KILLER_MAX];
   Depth reduction;
 };
 
@@ -66,10 +68,9 @@ struct SearchStack {
 //// Global variables
 ////
 
+extern SearchStack EmptySearchStack;
 extern TranspositionTable TT;
-
 extern int ActiveThreads;
-
 extern Lock SMPLock;
 
 // Perhaps better to make H local, and pass as parameter to MovePicker?