Well, I think both of them should be added into Guile. :-)
I confess 'file-system-fold' is more hackable and interesting, but it
contains 6 args.
I don't mean 6 args is  improper. But maybe some guys just want an easy
thing, and maybe Guile newbies need a easy start with 'scandir'.
'scandir' is a POSIX function, so many people will think it exists in Guile.
We have 'opendir' and 'readdir', but no 'scandir', that's strange.