RFC: remove native types

We have unsafe pointers, and we have zero-variant enums. This seems a good alternative for opaque pointers:

```
enum foreign_object {}
fn do_foreign_things(o: *foreign_object) { ... }
```

For non-opaque things, just using `*u8` or `*record_type` is also clearer than using a native type.

Please weigh in if you think they have some merits and should be kept.
