pub fn par_eval_map<I, O, Fut, F>( s: RS2Stream<I>, concurrency: usize, f: F, ) -> RS2Stream<O>where F: FnMut(I) -> Fut + Send + 'static, Fut: Future<Output = O> + Send + 'static, O: Send + 'static, I: Send + 'static,
Parallel evaluation preserving order (parEvalMap) with automatic backpressure