Add tf.train.Checkpoint for reading and writing object-based checkpoints.

Previously exposed as tf.contrib.eager.Checkpoint / tfe.Checkpoint.

Spiffies up the documentation a bit, but otherwise just adds the export decorator.

Compatible in both directions with tf.train.Saver (object-based checkpoints can be fed to tf.train.Saver, and name-based checkpoints can be fed to tf.train.Checkpoint).

PiperOrigin-RevId: 193439442