Improve 'board::BitboardIterator'

* Accurate 'size_hint'.
* Exact size.
* Fused iterator (keeps returning 'None' after returning 'None' once).

src/board/bitboard/iterator.rs

@@ -14,4 +14,14 @@ impl Iterator for BitboardIterator {
             Some(crate::board::Square::from_index(lsb))
         }
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let size = self.0.count_ones() as usize;
+
+        (size, Some(size))
+    }
 }
+
+impl ExactSizeIterator for BitboardIterator {}
+
+impl std::iter::FusedIterator for BitboardIterator {}

src/board/color.rs

@@ -1,4 +1,4 @@
-use super::Rank;
+use super::{Direction, Rank};
 
 /// An enum representing the color of a player.
 #[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
@@ -65,6 +65,21 @@ impl Color {
             Self::Black => Rank::Second,
         }
     }
+
+    /// Which way do pawns advance for this color.
+    #[inline(always)]
+    pub fn forward_direction(self) -> Direction {
+        match self {
+            Self::White => Direction::North,
+            Self::Black => Direction::South,
+        }
+    }
+
+    /// Which way do the opponent's pawns advance for this color.
+    #[inline(always)]
+    pub fn backward_direction(self) -> Direction {
+        (!self).forward_direction()
+    }
 }
 
 impl std::ops::Not for Color {