Replace clear_bit() with xor_bit()

This allows to retire ClearMaskBB[] and use just
one SquareBB[] array to set and clear a bit.

No functional change.

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

src/types.h

@@ -333,6 +333,7 @@ const Value QueenValueEndgame  = Value(0x9FE);
 extern const Value PieceValueMidgame[17]; // Indexed by Piece or PieceType
 extern const Value PieceValueEndgame[17];
 extern int SquareDistance[64][64];
+extern uint8_t BitCount8Bit[256];
 
 inline Color operator~(Color c) {
   return Color(c ^ 1);