BREAKING: `arti_client::TorClient<R>::launch_onion_service()` and `arti_client::TorClient<R>::launch_onion_service_with_hsid()` now return `Result<Option<(Arc<tor_hsservice::RunningOnionService>, impl futures::Stream<Item = tor_hsservice::RendRequest> + use<R>,)>` (the `Option` is new).
