[bitboard] Implement BitBoard::occupied_squares_direction

Iterate a BitBoard in a direction (from leading or trailing edge) based on a
board direction.

bitboard/src/bitboard.rs

@@ -233,6 +233,38 @@ impl BitBoard {
         }
     }
 
+    /// Iterate through the occupied squares in a direction specified by a
+    /// compass direction. This method is mose useful for bitboards of slider
+    /// rays so that iteration proceeds in order along the ray's direction.
+    ///
+    /// ## Examples
+    ///
+    /// ```
+    /// use chessfriend_bitboard::BitBoard;
+    /// use chessfriend_core::{Direction, Square};
+    ///
+    /// let ray = BitBoard::ray(Square::E4, Direction::North);
+    /// assert_eq!(
+    ///     ray.occupied_squares_direction(Direction::North).collect::<Vec<Square>>(),
+    ///     vec![Square::E5, Square::E6, Square::E7, Square::E8]
+    /// );
+    /// ```
+    ///
+    #[must_use]
+    pub fn occupied_squares_direction(
+        &self,
+        direction: Direction,
+    ) -> Box<dyn Iterator<Item = Square>> {
+        match direction {
+            Direction::North | Direction::NorthEast | Direction::NorthWest | Direction::East => {
+                Box::new(self.occupied_squares_trailing())
+            }
+            Direction::SouthEast | Direction::South | Direction::SouthWest | Direction::West => {
+                Box::new(self.occupied_squares_leading())
+            }
+        }
+    }
+
     #[must_use]
     pub fn occupied_squares_leading(&self) -> LeadingBitScanner {
         LeadingBitScanner::new(self.0)