Remove private types

[senier]

The generator currently doesn't support private types (cf. #758). We need to decide whether we want / need private types at all an either implement them in the generator or remove them from the language.

[treiher]

As we don't have a specific use case for private types, I would be in favor of removing them.

[senier]

With the introduction of tagged types this is not necessary anymore. We will remove private types.