pub fn board_fen(board: &Board) -> String
Create a board FEN such as rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR with default FenOpts.
rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR
FenOpts