Expose API to retrieve string representation of KeyPath
We can build a type-safe NSPredicate using KeyPath if we have such an API. We can use `_kvcKeyPathString` currently, but it would be nice if it will be officially visible as a public API.
For example, I created this experimental project using KeyPath and `_kvcKeyPathString`.
Using this extension, you can type-safely write queries that were previously hard-coded with strings as shown below. In addition to type checking, a typo can be prevented, and code completion works.