because the former doesn't create a reducer that handles kv-reduction?
If so, it seems that `folder`/`reducer` are not fns intended for public consumption, since `rfn` is private and there's no easy way to create reducing function transformers that work with `folder`/`reducer`. Is this the case?
Creating a reducer from a transducer may not be very useful, given that `reduce`/`transduce` can be used to get sequential reduction either way. Creating a folder from a transducer, however, seems like it'd be useful--the transducer captures the what, while the folder captures the how (parallel via fork/join). So it'd be nice if
was the way to do it.