Investigate struct upgrading delegate method

[nickoneill]

To give users a way to modify struct property names or do other upgrading on a struct, we should have some optional method you can implement to tell Storage what to do with certain keys (i.e. "name" is now "fullName", unarchive to the new property)

• do we need to store a struct hash here to track struct version changes?

Not on my short list of things to do, but something to think about.