C API: Use opaque structs instead of void

[roberth]

Is your feature request related to a problem? Please describe.

• Currently the C compiler will not catch certain errors such as switched arguments, because all void * typedefs are assumed interchangeable.
• Similarly, generated bindings forward the problem.

Describe the solution you'd like

Use forward declarations.

struct nix_foo;
typedef struct nix_foo nix_foo;

Describe alternatives you've considered

Maybe some bindings generator had an annoying behavior, compared to void pointers? I find this unlikely.

Additional context

Priorities

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