[board] Export BitBoard directly off of the crate for internal use

Update all the imports to import from the crate directly.

board/src/lib.rs

@@ -12,3 +12,5 @@ mod square;
 pub use moves::Move;
 pub use position::Position;
 pub use square::{File, Rank, Square};
+
+pub(crate) use bitboard::BitBoard;