Add board function for retrieving indexes of a piece

Hi! My name is Justus Karlsson. Please add this feature 🤓 Functionality of requested function:

        chess::bitboard pawns = board.side_piece_mask(side, chess::piece::piece_pawn);
        int i = 0; // A1
        // Make issue of this
        while(pawns > 0) {
            if (pawns & 1) {
                // Add i to indexes
            }
            pawns >>= 1;
            i++;
        }