proposal concerning conflicts between nset and other stuff in Maxima



I agree about moving these functions, but I'd also suggest renaming
them so that nset can be used along with these share packages.

> (1) Move src/flatten.lisp to share/algebra/charsets/.
>     ($flatten is the only Maxima function defined in flatten.lisp.)
> 
> (2) Cut $every from src/mutils.lisp and paste it into
>     share/algebra/charsets/flatten.lisp
>     (I guess pasting it into share/algebra/charsets/set.lisp
>     would work just as well.)