Examine common migration patterns

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Users will often want to expand Plate schemas while still keeping backwards compatibility with old data. Lets examine some of the ways this can play out.

Adding a new field to a sum type

100% backward compatibility with old data. Nice.

Adding a new field to a product type

Currently this always breaks backwards compatibility. We could avoid some of this in two ways: either by allowing optional fields to be missing or by allowing defaults to be set. This ties in to issue #2.