Support structs that have a private inner type

This currently runs afoul of private-in-public rules.

```rust
#[macro_use]
extern crate ref_cast;

struct T;

#[derive(RefCast)]
pub struct U(T);
```

```
error[E0446]: private type `T` in public interface
 --> src/main.rs:6:10
  |
6 | #[derive(RefCast)]
  |          ^^^^^^^ can't leak private type
```