Prototype for object-based save/restore

Includes deferred restoration (mostly useful for eager execution).

Slot variables are created with their checkpointed values as soon as the variable they're slotting for is restored, so there's no need to override slot creation (optimizers already check for existing slot variables before creating a new one).

Changes the behavior of unnamed Checkpointable dependencies so that only other unnamed dependencies can interfere (named dependencies do not get a number). This should be a bit more robust, and will support property assignment syntax sugar in a future CL. It does mean that removing a name from a dependency will break the checkpoint (just like changing its name would).

One minor fix for slot creation eager compatibility.

PiperOrigin-RevId: 184871747