[position, perft] Move Perft into the position crate

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.

position/src/lib.rs

@@ -9,5 +9,6 @@ pub use chessfriend_board::{fen, PlacePieceError, PlacePieceStrategy};
 pub use chessfriend_moves::{GeneratedMove, ValidateMove};
 pub use position::Position;
 
+pub mod perft;
 #[macro_use]
 pub mod testing;