Add stable-set-difference

[Ambrevar]

set-difference is a notorious pitfall in Common Lisp since it does not guarantee the order of the returned elements, which is arguably counter-intuitive.

Thus I propose Serapeum includes a stable-set-difference function.

[ruricolist]

Good idea! Good name for it too. This is true of all the other set functions also. A stable-union is something I would reach for often.

[Ambrevar]

@ruricolist Thanks a lot for the prompt implementation @aartaka It's now in! :)

[aartaka]

Hooray!