[board] Reorganize bitboard and position modules and export some symbols from the crate

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.

board/src/lib.rs

@@ -3,3 +3,7 @@ mod neighbor;
 mod piece;
 mod position;
 mod square;
+
+pub use piece::{Color, Piece};
+pub use position::Position;
+pub use square::Square;