I'm looking to move towards a tree-like structure for dirty checking
that involves an attribute holding onto the attribute that it was
created from. This means that `changed?` can be fully encapsulated on
that object. Since the objects are immutable, in `changes_applied`, we
can simply perform a shallow dup, instead of a deep one.

I'm not sure if that will actually end up in a performance boost, but
I'd like to semantically separate these concepts regardless