- Rename `AstKind` to `AstParentKind`.
- Add `kind()` to `AstNodeRef`.
- Reduce generic arguments of `AstNodePath`.
- Remove `Opt*` from `AstParentKind` and `AstNodeRef`.
- Add field enum for structs.
- Add field enum for enums.
- Change `Vec*` to a child kind.
- Add field for the child field to `AstParentKind`.
- Add field for the child field to `AstNodeRef`.