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I've brought the patch up to date.  I'm not sure if we
really need this patch.  The current approach works okay.
I think this patch improves things since more
details about the GC implementation are hidden.  This would
give us more freedom to change the implementation in the
future.  Also, I think the patch makes it easier for
extension types to support GC.  The patch is somewhat
backwards compatible now.  Types that use the old interface
will still compile but will not be GCed.