Expose EvalInfo struct to search

Allow to use EvalInfo struct, populated by
evaluation(), in search.

In particular we allocate Eval::Info on the stack
and pass a pointer to this to evaluate().

Also add to Search::Stack a pointer to Eval::Info,
this allows to reference eval info of previous/next
nodes.

WARNING: Eval::Info is NOT initialized and is populated
by evaluate(), only if the latter is called, and this
does not happen in all the code paths, so care should be
taken when accessing this struct.

No functional change.

src/search.h

@@ -25,6 +25,7 @@
 #include <stack>
 #include <vector>
 
+#include "evaluate.h"
 #include "misc.h"
 #include "position.h"
 #include "types.h"
@@ -48,6 +49,7 @@ struct Stack {
   Value evalMargin;
   int skipNullMove;
   int futilityMoveCount;
+  Eval::Info* ei;
 };