Support nested values

As mentioned on the readme, nested values are not supported yet. They would have to be encoded as `CKReference`.

Start by reading the README section that says nested values are unsupported and locate the existing CKRecord encoding and decoding entry points. Determine how nested values should be represented as CKReference, then add coverage for the intended behavior; the issue does not name a file or test, so the completion criteria need clarification.

由索引模型根据 Issue 内容生成。