gc tweaks

[Matthew Fluet]

> > Maybe my coffee just hasn't kicked in yet, but how does `share' work?
> > Some kind of common-subobject-elimination?  It just seems that the
> > equality test would be prohibitively expensive.
>
> I assume it uses hash consing, where mutable objects are always not equal.

That makes sense.  I guess it depends whether "maximal sharing" is a true
maximum.

There also seems to be a question of memory usage.  I.e., I'm running low
on heap space, so in addition to GCing, I also want to maximize sharing;
but that requires additional (and possibly unavailable) space to store the
hash consing table, etc.  Maybe it's a non-issue.