Remove usage of unstable box syntax, except in the script crate

… because there’s a lot of it,
and script still uses any other unstable features anyway.

components/devtools/actors/memory.rs

@@ -45,7 +45,7 @@ impl MemoryActor {
             name: actor_name.clone()
         };
 
-        registry.register_later(box actor);
+        registry.register_later(Box::new(actor));
         actor_name
     }