DEV: Separate FileCheckpointManager and GenericFileCheckpointManager. - 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.