Fixes#7706
We previously had a very specific backwards compatibility handler for
handling renamed fields. We always knew that wouldn't scale but I wanted
to wait for a real case. Well, #7706 is a real case, so here we are.
This commit makes our backwards compatibility handler more general
purpose, and makes a special-case handler for renamed fields built on
top of this same general purpose system. The new system lets us do a lot
more with regards to backwards compatibility.
To start, this addresses #7706 by allowing us to handle a removed single
enum value of a still-existing field.
84432a7beb