Add rewind functionality only (no UI)

Not active without changing code for now, but it works.  I think I was shown an example where it actually visibly rewound, which this won't, but it allows stepping back N checkpoints (where N is currently 5, and their frequency is not yet defined.)

If a savestate load from file operation fails, also tries to restore the rewind snapshot before resetting the game entirely.  This means they may likely not lose too much (assuming the feature is enabled.)

-[Unknown]