Dealing with issues from dividing an API over multiple files.

[All8Up]

PR #210 is some initial work towards this.
Basically there are two cases being handled for the specific need, though done fairly generically:

1. I want to extract the error type out of the APIs and 'include' it in C. The error is an enum and Interopto didn't have an equivalent "Opaque" type for that case.
2. I have a case where I'm including 3rd party headers for specific types and Interopto was recasing the types. I needed the types to stay the same case.

This seemed like a reasonable approach though I'm not 100% sure there was not some more simplified solution, please let me know.

[ralfbiedert]

While the C backend is currently defunct in 0.16 alpha, there is much better support in Interoptopus core now for fine-grained item manipulations via RustInventory, the ID system, and the Emission marker.

If / when the C backend is done similar to the C# backend, something like this should be supported as well.