Individually mark struct type as opaque.

`dependency-only: opaque` is useful.

But I have some edge cases where I actually want some structs which are only exposed as pointers, but not others. I am just generating all of them now.

This is not a big problem tbh. But from code size perspective, would be nice to mark a struct manually as opaque.

I have also considered just adding a function which returns the struct (not-pointer). But it gets unweildy if you only want few structs as opaque.

I don't know if this a common problem or unique to my use case.