When a knight has traveled around a chessboard touching every square exactly once, it has complted a "knight's tour."
Your task is to complete a knight's tour on the board below. Green squares indicate a legal move, and the numbers on each
square tell you how many open moves are available from that square. The "Warnsdorff" heuristic for finding a tour is to
always move to a square with the lowest number of available moves.