pub fn cycle_route_preloaded(&self, keep_urls: &[&PathMaybeWithLocale]) {
        let mut preloaded = self.route_preloaded.borrow_mut();
        preloaded.retain(|url, _| keep_urls.iter().any(|keep_url| *keep_url == url));
    }
Clears all the routes that were preloaded for the last route, keeping
only those listed (this should be used to make sure we don't have to
double-preload things).
packages/perseus/src/state/state_store.rs