[position] Implement a SliderRayToSquare trait

This trait declares ray_to_square() which should return a BitBoard representing
a ray from a Square to another Square. The ray should include the target Square.

PlacedPiece implements this trait.

bitboard/src/bitboard.rs

@@ -39,7 +39,7 @@ impl BitBoard {
         library::FILES[*file as usize]
     }
 
-    pub fn ray(sq: Square, dir: Direction) -> BitBoard {
+    pub fn ray(sq: Square, dir: Direction) -> &'static BitBoard {
         library::library().ray(sq, dir)
     }
 
@@ -107,6 +107,24 @@ impl BitBoard {
     pub fn occupied_squares_trailing(&self) -> impl Iterator<Item = Square> {
         LeadingBitScanner::new(self.0).map(|idx| unsafe { Square::from_index(idx as u8) })
     }
+
+    pub fn first_occupied_square(&self) -> Option<Square> {
+        let leading_zeros = self.0.leading_zeros() as u8;
+        if leading_zeros < Square::NUM as u8 {
+            unsafe { Some(Square::from_index(Square::NUM as u8 - leading_zeros - 1)) }
+        } else {
+            None
+        }
+    }
+
+    pub fn first_occupied_square_trailing(&self) -> Option<Square> {
+        let trailing_zeros = self.0.trailing_zeros() as u8;
+        if trailing_zeros < Square::NUM as u8 {
+            unsafe { Some(Square::from_index(trailing_zeros)) }
+        } else {
+            None
+        }
+    }
 }
 
 impl Default for BitBoard {
@@ -377,4 +395,14 @@ mod tests {
         assert_eq!(BitBoard::from(Square::A1), BitBoard(0b1));
         assert_eq!(BitBoard::from(Square::H8), BitBoard(1 << 63));
     }
+
+    #[test]
+    fn first_occupied_squares() {
+        let bb = bitboard![A8, E1];
+        assert_eq!(bb.first_occupied_square(), Some(Square::A8));
+        assert_eq!(bb.first_occupied_square_trailing(), Some(Square::E1));
+
+        let bb = bitboard![D6, E7, F8];
+        assert_eq!(bb.first_occupied_square_trailing(), Some(Square::D6));
+    }
 }

bitboard/src/library.rs

@@ -220,8 +220,8 @@ impl MoveLibrary {
         ray
     }
 
-    pub(super) fn ray(&self, sq: Square, dir: Direction) -> BitBoard {
-        self.rays[sq as usize][dir as usize]
+    pub(super) fn ray(&self, sq: Square, dir: Direction) -> &BitBoard {
+        &self.rays[sq as usize][dir as usize]
     }
 
     pub(super) fn pawn_pushes(&self, sq: Square, color: Color) -> BitBoard {