Implement MAP-REFERENCING-OBJECTS, and make the implementation
of LIST-REFERENCING-OBJECTS use it.
... note potential badness of cutoff point in space search.
(could this be the problem in ROOM T?)
Improvements to stale-symbols script.
... if there is more than one reference, don't bother chasing
all the others down;
... closures can contain stuff;
... print symbols and references a little more nicely.