[position, board] Move castle, movement, and sight modules to the board crate

Nothing about this code depends on Position, so push it down to a lower layer.

board/src/lib.rs

@@ -5,6 +5,8 @@ pub mod display;
 pub mod en_passant;
 pub mod fen;
 pub mod macros;
+pub mod movement;
+pub mod sight;
 
 mod board;
 mod piece_sets;