Revise fixup-features to be closer to something we can use
We have a work-in-progress program called "fixup-features" that is meant to enforce various rules on the features in our crates. The trouble is, we've never run it successfully: The one time we tried, we ran into trouble. (See !1068 (closed))
This branch tries to rewrite "fixup-features", clean it up a tiny bit, and make its behavior closer to what we really need.
Notes: