bonzai/vars pollutes XDG_STATE_HOME directory with empty cache

[Chaitanyabsprip]

No description provided.

[rwxrob]

We really have to look at bonzai/vars fresh now that we have the other variable persistence stuff and the collection of bonzai/persisters (one of which is most of what vars uses right now). Let's let the recent persistence/inheritance/declaration variable pr merge settle for a while and then rethink what the vars command should do. I would probably like to make it so that people can decide what type of structured data to use and where to store it, which we have now with the bonzai.Persister interface.