Add a save method to the _Model protocol

[Obbut]

This PR adds a save method to the _Model protocol. It also adds a default implementation of the method to the Model protocol.

Because the _Model protocol is not meant to be directly conformed to, this change can be considered as purely additive and non-breaking.

Motivation

The Model protocol has an associated type requirement (Identifier). There are cases where you want to perform a save operation on a Model, while the exact type is unknown. With this PR, you can:

let maybeAModel: Any = ...
if let model = maybeAModel as? _Model {
    model.save(to: context)
}