Remove need to box kernel on model

examples/kickscore-basics.rs

@@ -21,9 +21,9 @@ fn main() {
     ];
 
     // Now we are ready to add the items in the model.
-    model.add_item("Spike", Box::new(k_spike));
-    model.add_item("Tom", Box::new(k_tom));
-    model.add_item("Jerry", Box::new(k_jerry));
+    model.add_item("Spike", k_spike);
+    model.add_item("Tom", k_tom);
+    model.add_item("Jerry", k_jerry);
 
     // At first, Jerry beats Tom a couple of times.
     model.observe(&["Jerry"], &["Tom"], 0.0);