[bitboard, board] Replace ray_in_direction! macro with a function

This is simpler than writing a macro, at the expense of some overhead for calling
a function. But the Rust compiler might inline it anyway!

To support this change, implement BitBoard::first_occupied_square_direction, which
iterates a bitboard in a direction (i.e. leading or trailing) depending on the
core::Direction value passed to it.

bitboard/src/bitboard.rs

@@ -243,6 +243,18 @@ impl BitBoard {
         TrailingBitScanner::new(self.0)
     }
 
+    #[must_use]
+    pub fn first_occupied_square_direction(&self, direction: Direction) -> Option<Square> {
+        match direction {
+            Direction::North | Direction::NorthEast | Direction::NorthWest | Direction::East => {
+                self.first_occupied_square_trailing()
+            }
+            Direction::SouthEast | Direction::South | Direction::SouthWest | Direction::West => {
+                self.first_occupied_square_leading()
+            }
+        }
+    }
+
     #[must_use]
     pub fn first_occupied_square(&self, direction: &IterationDirection) -> Option<Square> {
         match direction {