Storage interface: Extract the static and instance methods from `persist`.

Static methods are accessible via `PolyClay.persist.statics`; instance
methods via `persist.plugins`. This simplifies overriding and extending the
storage methods of model functions, and reduces the overhead of constructing
the storage functions anew upon invoking `persist`.
Merge pull request #13 from ceejbot/persistence-extraction

Storage interface: Extract the static and instance methods from `persist`.