GC the RuleTree when the Stylist is dropped.
GC the RuleTree when the Stylist is dropped.

We only consider doing a GC currently if the root node has a zero refcount.
But that only happens if it has no children -- even weak children keep a
strong reference to their parent.  So at the very least, we should do a
GC specifically when the RuleTree is going away.  (We probably want to add
some other GC opportunities too at some point, otherwise it's easy to
never GC a RuleTree.)

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