At the first level of depth, print the move and the number of nodes counted in
the tree underneath that node. This behavior imitates Stockfish, and helps with
debugging.
Clean up the output of the Perft binary, and update the check-positions script
to compensate.
Move the Perft trait into the position crate, and let the perft binary call into
that.
Amend Position::make_move to return a bool in the Ok case that indicates whether
the position has been seen before. Use this to decide whether to continue
recursing during the Perft run. I haven't seen that this makes a difference in
the counts returned by Perft yet.
