Use Blender Containers in the AbstractHierarchyIterator system. A future
bug fix in the area is looking like it will require another map so it
seems like a good time to replace the old containers before that lands.
The conversion is complicated due to how the prior code leveraged
`operator[]` to both add and update entries; it was never clear which
operation was being performed. This leads to a somewhat confusing mix
of `add_new`, `add`, and `lookup_or_add` calls. The calls in place now
are what was required based on our existing tests.
Additionally, pointer stability guarantees were a little bit different,
affecting the usage of the `graph_children` function the most.
Pull Request: https://projects.blender.org/blender/blender/pulls/134698
6704647c66