Add support for testing objects for equality

[ftchirou]

Fixes #14 and #19. Adds support for testing equality of objects conforming to Identifiable with the == operator in predicates.

Example

class Note: NSManagedObject {
  @NSManaged var attachment: Attachment
}

class Attachment: NSManagedObject, Identifiable {
  @NSManaged var id: String
  ...
}

let attachment: Attachment = // ...
let predicate = \Note.attachment == attachment