Support opaque types that are not structs

[dtolnay]

Fixes #13.

This way we can have:

// c++

using Obj = void*;

// rust

#[cxx::bridge]
mod ffi {
    extern "C" {
        include!("...");

        type Obj;
        fn f(obj: &Obj);
    }
}

We no longer assume anything about opaque C++ types except that something with the right name has been declared by one of the header(s).

cc @not-a-seagull

[notgull]

Well you're fast... 😄

I think this would solve the problem I've been having.

[dtolnay]

I published 0.1.2 containing this fix.