`cddl-derive` recursive types have infinite size

When two CDDL rules reference each other cyclically, `cddl-derive` emits code that errors in Rust. Ideally it should box one of the types deterministically.

Repro:
```cddl
node = {
  ? "name": tstr,
  ? "child": child,
}

child = {
  ? "value": int,
  ? "parent": node,
}
```
Error: ```recursive types `Node` and `Child` have infinite size```