Expose Checkpointable symbols in tf.contrib.eager/tfe

- tfe.Checkpoint
    Utility for grouping Checkpointable objects into training checkpoints, has
    save/restore methods which call CheckpointableSaver.
- tfe.Checkpointable
    For user-defined Checkpointable objects.
- tfe.CheckpointableSaver
    More control over saving/restoring than tfe.Checkpoint.

Only tfe.Checkpoint is required to switch examples over, so I can leave the
others out if there are objections.

PiperOrigin-RevId: 187555472