pub fn filter_map<T, U, F, Fut>(s: RS2Stream<T>, f: F) -> RS2Stream<U>where F: FnMut(T) -> Fut + Send + 'static, Fut: Future<Output = Option<U>> + Send + 'static, T: Send + 'static, U: Send + 'static,
Filter and map elements of a stream in one operation