code: public <R> Future<R> map(final Function<? super T, ? extends R> fn, Executor ex) {
          return new Future<R>(
                                future.thenApplyAsync(fn,ex));
      }
docs: Asyncrhonous transform operation
  
  @see CompletableFuture#thenApplyAsync(Function, Executor)
  
  @param fn Transformation function
  @param ex Executor to execute the transformation asynchronously
  @return Mapped Future