Or the bishop/rook and queen masks instead of and.

Fix issue where double pawn pushes were not generated if single push was impossible.
This occured when the double pawn push blocked a check, so the single push was not legal.

En Passant pins are tricky because two pieces vanish from the same rank
together. Properly deal with them using some original bitboard trickery.

Code now uses the SeenByEnemy bitboard to check if the king goes through
check while castling. Defined some new bitboards to aid the checking.

En Passant captures are now generated separately from normal attacks.
Also fixed the issue where ep was not considered valid when the ep pawn
was the king's checker.