Add new multiset primitives

[saulshanabrook]

WIP to add new multiset primitives. Used in egraphs-good/egglog-python#397 to try avoiding AC blowup using multisets as polynomials.

Also supports automatically registering unstable function primitives when other sorts are created. Now we can do this:

(sort Math)
(sort Maths (MultiSet Math))
(sort MathToMath (UnstableFn (Math) Math))

Instead of:

(sort Math)
(sort MathToMath (UnstableFn (Math) Math))
(sort Maths (MultiSet Math))

and the unstable-multiset-map primitive will still exist.

This is needed for some of the new primitives, to avoid cyclic sort ordering requirements.