Allow pawn to upgrade to other pieces #16
Description
A pawn when hitting the end of the board, may choose to promote to any piece (excluding king and pawn). So technically this move should create a board for each of these other pieces.
I think it would be useless to generate a rook or bishop over a queen, but a knight may be useful, so our move generator should create 2 boards for each promotion (1 queen and 1 knight)