Rename Feature Meta/Virtual to Definition

Mark, I know this actually the opposite of what we talked about, but I figured this would be nicer to pattern match on and when you need `ConcreteDefinition` directly it has more context. Win/win. :)

We definitely have too many tests that should be using `Arbitrary`, but I don't want to get bogged down fixing them now.
> Can you chuck a card in easy picking to either delete or move generate to its own project and make it more useful.

Done