iterator fixes

src/board.rs

@@ -74,6 +74,14 @@ impl Board {
         self
     }
 
+    pub fn possible_moves(&self, color: Piece) -> Box<dyn Iterator<Item = (usize, usize)> + '_> {
+        Box::new(
+            (0..BOARD_SIZE)
+                .flat_map(|i| (0..BOARD_SIZE).map(move |j| (i, j)))
+                .filter(move |(i, j)| self.what_if(*i, *j, color).is_some()),
+        )
+    }
+
     /// Returns a mutable reference to a place on the [`Board`]
     /// at (i, j)
     pub const fn get_mut(&mut self, i: usize, j: usize) -> &mut Option<Piece> {