Enforce feature conventions in all our crates (for cargo-semver-checks, and in general)

If we want to use cargo-semver-checks (see #711) and behave correctly wrt all our features, then either we have to wait for Cargo to support marking features as unstable, or we need to get our full feature working a bit more consistently.

I think that to do this properly, we should have a script that enforces all of these:

• Every one of our crates has a "full" feature.
• Every feature in our crates is reachable from full or experimental or from a third new __nonadditive feature. (The latter would exist only for feature flags like static libraries that do not belong in full because they are not strictly additive.)
• No feature is reachable from more than one of those.
• Our full feature, in each crate, enables full in all of our crates that it depends on. (I'm not sure about this one, but I think it is necessary for us to get the behavior we want out of cargo-semver.)

This is the sort of thing we could check with a tool or a script: ideally it would also edit our toml files for us, since some of these changes are nontrivial but mechanical.

Open questions: Is this worthwhile? How much of a priority should it be?