Remove PersistCore class

[parsonsmatt]

The PersistCore class defines the data family data BackendKey backend, and implementations use this to specify the default or implied key of an entity.

This is only useful in the case of mpsGeneric being true. Then, an Entity (Foo SqlBackend) will contain an entityKey :: Key (Foo SqlBackend), which (by default) will be BackendKey SqlBackend. Meanwhile, an Entity (Foo MongoContext) will contain a Key (Foo MongoContext), which is a newtype around Mongo.ObjectId.

If removing mpsGeneric #1204 is feasible, then we can also remove PersistCore as a class. Then, BackendKey becomes a plain type family, and it can point to plain newtypes.