Registry refactor and clear queues on board entry

kiln-core/src/board.rs

@@ -585,6 +585,15 @@ impl Board {
     fn terrain_layer_at(&self, x: usize, y: usize) -> Option<usize> {
         (0..self.layers.len()).find(|&z| self.get(z, x, y).1 != Archetype::Empty)
     }
+
+    /// Clear the queues of all objects on this board: called when entering a board, objects
+    /// don't retain their state across board visits (they get initialized again, but can
+    /// store things in the board registry)
+    pub fn clear_all_queues(&mut self) {
+        for obj in self.objects.values_mut() {
+            obj.queue.clear()
+        }
+    }
 }
 
 #[cfg(test)]