Abstract children list and dict collections into a class

[aedwardstx]

Changes

• Abstract the two children list and dict collections into a single class.
• Relax pydantic pinning a little.
• In _difference(), consider sectional_overwrite first and do not addsubtree just to delete it.
• Add return_if_present and check_if_present args to add_child().
• Simplify call to config_to_get_to().
• Rework __eq__() methods of HConfig related objects.
• Use TypeVar to remove the need to do 3 isinstance() checks.
• Remove need to do a second children.get() in add_children_deep().
• Simplify overwrite_with() logic.
• Various test improvements.