wanted: who-references and who-sets for clos slots

[Tobias C. Rittweiler]

It would be nice to have xref data for the locations where CLOS slots are referenced
and where they're written to. I'm thinking to track the relevant call sites of SLOT-VALUE
and (SETF SLOT-VALUE) here.

Information about where accessor functions are used is currently available via
WHO-CALLS. It may be nice to differentiate between calls to ACCESSOR-FN
as references, and calls to (SETF ACCESSOR-FN) as sets -- and to include those
locations in the output of WHO-REFERENCES and WHO-SETS.

I don't feel strong about the latter, though, as its information is already available.
I'm more interested in the former point because its functionality is currently simply
missing.

[Nikodemus Siivola (nikodemus)]

 status confirmed
 importance wishlist
 tag feature

While I'm by no means opposed to this, I think getting this right is pretty tricky due to slot-value optimizations. Aspiring PCL hackers are welcome to attack this, because I'm probably not going to.