Add a functional saver, use it for object-based checkpointing

Pulls some utilities out of saver.py which are necessary to actually use it. The functional saver takes only SaveableObjects, so these are utilities for taking a list of whatever users pass in and converting them to those.

One other code move for object-based checkpointing to avoid circular imports.

Applications which need a SaverDef still use the old Saver. Serialization to SaverDef will be added to this saver in a followup.

Does not actually wrap the new Saver's methods in @tf.function yet, since there are memory issues which need to be fixed first.

PiperOrigin-RevId: 224561069