Change repo for fset

[death]

It seems the most up-to-date repository for fset is at https://github.com/slburson/fset

[quicklisp]

This repo has both fset.asd and Code/fset.asd. Do you know which one is the one to use?

[eudoxia0]

From Code/fset.asd:

;;; This is an alternate .asd file that I use for development.  You probably
;;; want the one in the parent directory.