C API interop header structure is inconsistent with C++ API header structure.

[RubyNova]

I'm not sure how to solve this. I'm not even sure if we can.

At present I am holding off from restructuring the C API headers fully into the new namespace style, as @tannergooding has concerns about the C# P/Invoke generation if we do that. However by leaving it alone, it's becoming a bit disorganised. I'm wondering if there's something we can do here?