In GC cards that are pointed to conservatively, wipe out unused words.
The unused words would act as false roots pointing to other objects
which would then be unnecessarily retained.  This could hold down
arbitrarily large trees of objects.

This commit carries a basic verbose print controlled directly by a
variable that I plan to remove later when I'm convinced it works as
advertised for a larger audience.  Tested with my toy at work and
found no instability and no slowdown.  My testing also indicated that
correct conservative roots are much more common than false ones, in
fact I found all to be valid (tested by severing them and crashing).
So this fix goes a long way only retaining valid objects across the
system.

This commit does not include further heap tracing tools that I talked
about before that I might add separately when I find a way to not make
the code too ugly.  I found that memory retention was largely caused
by mistakes in user code, not directly by this underengineering.  But
you couldn't prove that while the GC was randomly retaining things on
its own.