Revert previous enum instantiation behavior and PersistableEnum trait from de5f1e5a93
Backed enums will be encoded as their case value. Non-backed cannot be encoded and no enums can be encoded at the root level.
Prohibit enums from implementing Persistable and Unserializable.
Enums serialize like PHP objects with a "name" property (BackedEnum instance will also have a "value") and thus become BSON documents. In order for a document to unserialize back to an enum, it must implement either Unserializable or Persistable. The bsonUnserialize() method serves no purpose for initialization, but it will still be invoked.
This introduces a PersistableEnum trait, which can be used for both Unserializable or Persistable implementations.
Co-authored-by: Sara Golemon <pollita@php.net>
Jeremy Mikola