Expose API to retrieve string representation of KeyPath

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.

Before

NSPredicate(format: "name == %@", "Katsumi")
NSPredicate(format: "age > %@", 20)

After

Query(Person.self).filter(\Person.name == "Katsumi")
Query(Person.self).filter(\Person.age > 20)

Query(Person.self).filter(\Person.name > 20) // Compile error
Query(Person.self).filter(\Dog.name == "John") // Compile error