Introduce AnonymousKeyPath

In this evolution pitch from the Swift forums, Shai Mishali suggests the introduction of a new kind of KeyPath - AnonymousKeyPath. The idea here is that the key path can be used without knowing the Root of the KeyPath, but rather only the Value constrained to it. Although it's an interesting suggestion, the discussion that follows the pitch indicates that the problem it solves can be solved in other, more effective ways.