tweak the totally-bogus arbitrary stack-squeezing heuristic to fix #2797

tweak the totally-bogus arbitrary stack-squeezing heuristic to fix #2797
In #2797, a program that ran in constant stack space when compiled
needed linear stack space when interpreted.  It turned out to be
nothing more than stack-squeezing not happening.  We have a heuristic
to avoid stack-squeezing when it would be too expensive (shuffling a
large amount of memory to save a few words), but in some cases even
expensive stack-squeezing is necessary to avoid linear stack usage.
One day we should implement stack chunks, which would make this less
expensive.