[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/piece.rs

@@ -238,6 +238,16 @@ impl PlacedPiece {
     pub fn square(&self) -> Square {
         self.square
     }
+
+    #[inline]
+    pub fn color(&self) -> Color {
+        self.piece.color
+    }
+
+    #[inline]
+    pub fn shape(&self) -> Shape {
+        self.piece.shape
+    }
 }
 
 #[cfg(test)]