[gclist] Why circular destructor dependencies occur.

[Charles Fiterman]

Persistant objects generally have destructiors.
Since presistant objects usually refer to other
persistant objects its easy to get destructor
dependcies.

This suggests a division between destructors
and finalizers that can be used. Destructors
destroy! They wreck the objects invariants
so it can no longer be used. A file reference
object has destructors which close it. Finalizers
clean up. They leave the objects invariants.

Keeping a division between destructors and
finalizers breaks most loops. Further uncooperative
collectors can use this difference.