Add support for comparing raw representables

[ftchirou]

Make it straightforward to use enumerations (and other raw representables) in predicates.

Example

class Note: NSManagedObject {
  // ...
  @NSManaged var type: NoteType
}

@objc enum NoteType: Int {
  case freeForm
  case structured
}

let predicate = \Note.type == .structured

Previously, this 👆predicate would have required NoteType to conform to Primitive to compile. Not anymore.