Make deep-update and variants support sets and possibly lists

Currently, we can only deep update dictionaries into dictionaries.

Currently, if the dict value is a list or set, the new dict value wholesale replaces the old value.

In principle, though, it should be possible to merge sets if the old and new dict value contain sets. This gives us the ability to extend what was present rather than wholesale replace it.

There are a couple use cases where this would be huge nice-to-have. It would be worth supporting it as optional behaviors: `merge_sets` and `extend_iterables`