code: public <T, R> Stream<R> map(Stream<T> stream, Function<? super T, ? extends R> mapper) {
          Callable<Stream<R>> callable = () -> stream.map(mapper);
          return ThreadMethods.forkJoinExecution(callable, concurrencyConfiguration, stream.isParallel());
      }
docs: Executes map on the provided stream. If the Stream is parallel, it is
  executed using the custom pool, else it is executed directly from the
  main thread.
  
  @param <T>
  @param <R>
  @param stream
  @param mapper
  @return