Rename yang.binding.{Identifiable,Identifier}

Names here overlap with yangtools.concepts, leading to potential confusion with users. Furthermore we have to have an explicit @SuppressFBWarnings on Identifier, as it has an explicit tie-in with yangtools.concepts.Identifier.

Rename yang.binding.Identifier to yang.binding.Key and yang.binding.Identifiable to yang.binding.KeyAware – which are relatively unique names (Key could be confused with java.security.Key).