Restore deterministic search state

Introduce helper function Search::reset() which clears all kind of search
memory, in order to restore a deterministic search state.

Generalize TT.clear() into Search::reset() for the following use cases:
- bench: needed to guarantee deterministic bench (ie. if you call bench from
interactive command line twice in a row you get the same value).
- Clear Hash: restore clean search state, which is the purpose of this button.
- ucinewgame: ditto.

No functional change.

Resolves #346

src/movepick.h

@@ -21,6 +21,7 @@
 #define MOVEPICK_H_INCLUDED
 
 #include <algorithm> // For std::max
+#include <cstring>   // For std::memset
 
 #include "movegen.h"
 #include "position.h"
@@ -43,6 +44,7 @@ struct Stats {
 
   const T* operator[](Piece pc) const { return table[pc]; }
   T* operator[](Piece pc) { return table[pc]; }
+  void clear() { std::memset(table, 0, sizeof(table)); }
 
   void update(Piece pc, Square to, Move m) {