Ignore dev-dependencies

[Manishearth]

Currently cbindgen in parse-deps mode seems to include dev-dependencies and build-dependencies. It would be nice if that could be avoided.

(In my case it means that we hit https://github.com/eqrion/cbindgen/issues/665 even though it's only used by criterion)

[emilio]

Yeah, that's silly. Should be trivial to fix, let me know if you're blocked on it or need it fast and I should be able to fix quick.

[Manishearth]

Not blocked on it at the moment, I'm writing manual bindings for now.

[jonhoo]

One challenge here will probably be that when you invoke cargo metadata, it downloads all dependencies regardless of how they're pulled in. So while we may be able to make cbindgen not parse dev-dependencies, we may still end up having to fetch them.