Implement a checker for stored ZObjects.
Every top-level ZObject stored in WikiLambda must have the type Z2/Persistent object.
(Optional: and only top-level ZObjects stored in WikiLambda may be of type Z2/Persistent object).
A Z2/Persistent object must have the following four keys and respective values (and only these for now):
Z1K1/type with the value "Z2" for Persistent object.
Z2K1/ID. The value of this must be a string that is the same as the page name. It must be of the format Zn with n being a natural number.
Z2K2/value. The value of this must be a well-formed ZObject.
Z2K3/label. For now we start with this being a simple string. This will be turned into a multilingual label very soon.