[board] Implement a PositionBuilder; refactor piece bitboards into a PieceBitBoards struct

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.

board/src/lib.rs

@@ -13,8 +13,9 @@ pub mod position;
 mod sight;
 mod square;
 
-pub use position::Position;
-pub use r#move::{Move, MoveBuilder};
+pub use piece::{Color, Piece};
+pub use position::{Position, PositionBuilder};
+pub use r#move::{MakeMoveError, Move, MoveBuilder};
 pub use square::{File, Rank, Square};
 
 pub(crate) use bitboard::BitBoard;