Record Serialization - Update

[Brian Goetz]

>> What about serialVersionUID?
> The serialVersionUID is allowed and taken into consideration ( sorry, I accidentally omitted this important point ). Setting the serialVersionUID is required, to be able to migrate from a record-like class to a record. Since the former and latter will likely have different default serialVersionUID’s, if not explicitly set ( which one is encouraged to do ).

So, what we'll need here is guidance for:
  - compiler writers, what to generate if no svuid is explicitly specified;
  - migrators, what to do when migrating from a class to a record.