[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.

position/src/lib.rs

@@ -1,8 +1,6 @@
 // Eryn Wells <eryn@erynwells.me>
 
-mod movement;
 mod position;
-mod sight;
 
 #[macro_use]
 mod macros;