Migrate iOS FFI interface to structs (or enums)

[ivnsch]

After #39 is cleared, migrate JSON based FFI to structs (in case directly enums are not possible) or enums.

The structs would be the same we're serializing to/from JSON.

It probably makes sense to reconsider the usage of HTTP status codes for everything. We probably should use custom status codes instead (1, 2, 3...) and pass networking related results in a nested struct (which would contain the HTTP status code).