Commit abbebf8e used __getattr__ to proxy Value methods called on
Record. However, that did not proxy operators like __add__ because
Python looks up the special operator methods directly on the class
and does not run __getattr__ if they are missing.
Instead of using __getattr__, explicitly enumerate and wrap every
Value method that should be proxied. This also ensures backwards
compatibility if more methods are added to Value later.
Fixes#533.
ea94c9cc45