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.
A small Python script that reads a JSON list of positions and their known Perft
node counts to a certain depth, then invokes the Perft program for each position
and validates the output.
Peter Ellis Jones shared such a JSON list on GitHub. Import that file.
