- Adds a `GenericCheckpointMixin` as a helper for implementing the two
boundary-traversing Checkpoint API methods, `create_checkpoint` and
`restore_checkpoint`.
- `GenericFileCheckpointManager` is implemented as a subclass of
`FileCheckpointManager` using `GenericCheckpointMixin`. Note that
this is the safe subtyping relationship because of method
signature *contra*variance: `FileCheckpointManager` accepts
`FileContentsManager` in its method signatures type, whereas
`GenericFileCheckpointManager` accepts any `ContentsManager`.
- Moved Checkpoint-related classes to their own files.
