Use a Direction enum for Square deltas

Currently the NORTH/WEST/SOUTH/EAST values are of type Square, but conceptually they are not squares but directions. This patch separates these values into a Direction enum and overloads addition and subtraction to allow adding a Square to a Direction (to get a new Square).

I have also slightly trimmed the possible overloadings to improve type safety. For example, it would normally not make sense to add a Color to a Color or a Piece to a Piece, or to multiply or divide them by an integer. It would also normally not make sense to add a Square to a Square.

This is a non-functional change.

src/bitboard.h

@@ -152,7 +152,7 @@ inline Bitboard file_bb(Square s) {
 
 /// shift() moves a bitboard one step along direction D. Mainly for pawns
 
-template<Square D>
+template<Direction D>
 constexpr Bitboard shift(Bitboard b) {
   return  D == NORTH      ?  b             << 8 : D == SOUTH      ?  b             >> 8
         : D == NORTH_EAST ? (b & ~FileHBB) << 9 : D == SOUTH_EAST ? (b & ~FileHBB) >> 7