[board] Implement piece sight algorithms

Add a new Sight trait, implemented by PlacedPiece. The implementation of this
trait produces a BitBoard representing the squares visible to the placed piece.

board/src/lib.rs

@@ -7,6 +7,7 @@ mod moves;
 pub mod piece;
 #[macro_use]
 pub mod position;
+mod sight;
 mod square;
 
 pub use moves::Move;