* Add `StableSortedListSet` and `StableSortedListMap` as memory-efficient alternatives to `SortedStrictSet`/`SortedStrictMap`.
* Speed up `SortedStrictSet::__unserialize` and `SortedStrictMap::__unserialize` when data is already sorted.
* Fix crash in `StrictSet` and `SortedStrictSet` during cyclic garbage collection.