Auto merge of #14167 - emilio:rule-tree-list, r=Manishearth

style: Don't assume siblings are alive in the rule tree when removing ourselves from the child list.

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We can't assume all our siblings are alive because they may very well be in the
free list too.

This tempts to happen when the rule nodes are destroyed as part of the last GC,
the one that runs in the root destructor.

Also, properly put the next sibling back into the list when the rules are GCd.

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