Top-level reference types

References at the top level should be possible. For example:

```
main = do
  let s: S = {- some complex deeply nested struct (but not enum!) -}
  [rustIO| () { println!("{:#?}", (&s: &S)) } |]
```

Under the hood, this should turn into a `Ptr S` which gets unsafely turned into a reference on the Rust side.