Restore to previous state with copy-on-write

Apologies if this isn't the right place for this kind of feature request.

As a Jupyter user, I would find it _incredibly_ helpful if there was an "undo" for kernels. Frequently, I run something, only to realize it was the wrong thing and I want to go back one cell. If the bad cell had any destructive effect, this means I need to restart my kernel and run all the cells above _again_. If those cells took 3 hours to run, I need to wait 3 hours again. Even if I don't think the bad cell had any destructive effect, if I fix up the notebook mid-stream others (and myself) won't be 100% sure that the notebook would work with a simple "restart and run all."

I wish there were a command like "Kernel -> Save Checkpoint" and then the ability to restore back to checkpoints. In Jupyter, if this worked in tandem with the checkpointing of the notebook file itself, it could create a seamless experience: I create a checkpoint, make a mistake, restore both kernel and notebook to the checkpoint, fix the mistake and keep running, and I end up with _exactly_ the notebook I would have had if I hadn't made the mistake in the first place. (On the Jupyter side I'd also like an option that automatically checkpointed after each cell, and one that forks off into a console instead of checkpointing the notebook file.)

Technically, it seems like this could be achieved through `fork`'s copy-on-write mechanism. For many notebooks that don't have much volatile state that is rewritten multiple times, even checkpoints after every cell could have near-zero memory overhead -- they would just be forks of the kernel process at that point, using the same pages in memory as the active kernel process unless they are subsequently modified.