Initial C API

[lu-zero]

To start discussing on the API itself:

• Function signatures dgc_{struct}_{action}(s: *mut Struct, args..., out: *mut *mut Out) -> *const ErrorType
• All the structs are opaque and they get accessors using the previous pattern

[lmammino]

Is there any way to auto-generate these?

I am coming with 0 expertise to this topic, but at first glance it would seem quite tedious and error-prone having to implement all of this manually and even worse having to keep things in sync over time...