Revert to byTypeBB[0] storing occupied squares

As it was in Glaurung times. Also rearranged order
so that byTypeBB[0] is accessed before byTypeBB[x]
to be more cache friendly. It seems there is even
a small speedup.

No functional change.

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

src/position.h

@@ -231,7 +231,6 @@ private:
   // Bitboards
   Bitboard byTypeBB[8];        // [pieceType]
   Bitboard byColorBB[2];       // [color]
-  Bitboard occupied;
 
   // Piece counts
   int pieceCount[2][8];        // [color][pieceType]
@@ -286,7 +285,7 @@ inline Color Position::side_to_move() const {
 }
 
 inline Bitboard Position::pieces() const {
-  return occupied;
+  return byTypeBB[ALL_PIECES];
 }
 
 inline Bitboard Position::pieces(Color c) const {
@@ -334,7 +333,7 @@ inline bool Position::can_castle(Color c) const {
 }
 
 inline bool Position::castle_impeded(CastleRight f) const {
-  return occupied & castlePath[f];
+  return byTypeBB[ALL_PIECES] & castlePath[f];
 }
 
 inline Square Position::castle_rook_square(CastleRight f) const {
@@ -354,11 +353,11 @@ inline Bitboard Position::attacks_from<PAWN>(Square s, Color c) const {
 }
 
 inline Bitboard Position::attacks_from(Piece p, Square s) const {
-  return attacks_from(p, s, occupied);
+  return attacks_from(p, s, byTypeBB[ALL_PIECES]);
 }
 
 inline Bitboard Position::attackers_to(Square s) const {
-  return attackers_to(s, occupied);
+  return attackers_to(s, byTypeBB[ALL_PIECES]);
 }
 
 inline Bitboard Position::checkers() const {