Add hinting feature to Fifteen (press 'h' for a hint).

Add hinting feature to Fifteen (press 'h' for a hint).

This is really an incremental solver. It alternates between solving
rows and solving columns. Each row and column is solved one piece at
a time. Except for some temporary trickery with the last two pieces
in a row or column, once a piece is solved it is never moved again.

(On non-square grids it first solves some rows or some columns until
the unsolved part is a square, then starts alternating.)