Alternative to parse-deps mode for opaque types

[Manishearth]

I keep hitting limitations of parse-deps: It fails to compile some crates (https://github.com/eqrion/cbindgen/issues/665), pulls in dev dependencies (https://github.com/eqrion/cbindgen/issues/666), and also seems to pull in proc macro dependencies (and it has a bad time with syn)

The weird thing is, at this moment all I need it for are opaque types. I just want to be able to use types from my dependencies behind raw pointers.

It would be nice if use statements could be enough for opaqueifying types. I know this leaves some ambiguity between trait objects and regular types and I'm not sure how to solve it, but it would be nice to have.