pub fn set<T: Any>(&mut self, name: String, thing: T) {
        let boxed: Box<dyn Any> = Box::new(thing);
        assert!(boxed.downcast_ref::<T>().is_some());
        self.handles.insert(name, boxed);
    }
Assign a thing to key `name`, boxed as `Box<Any>`.
server/src/lib.rs