This makes Position immutable, even if declared mut. I think this will also make
it easier to build positions going forward.
Moving to a PieceBitBoards struct also required a lot of places that return owned
BitBoards to return refs. This is basically fine except for adding a few more &
here and there.
This was a huge refactoring project. All the move generators and a bunch of
BitBoard stuff needed to be updated.
Move position.rs to the position module and create a mod.rs.
Do the same for bitboard.rs in the bitboard modules.
Export Color, Piece, Position, and Square and use crate::Thing directly instead of referring to the symbol in the nested modules.
