Make the Store & its backends support checkpointing

[sbazerque]

Right now, whenever obj.loadAllChanges() is called, all the mutations are loaded from the store and applied. It'd be very convenient to enable mutable types to save a checkpoint to the store when they so choose, and having loadAllChanges() automatically use the most recent checkpoint, having to apply only those mutations that happened after that point.

[sbazerque]

Started sketching an implementation:

https://github.com/hyperhyperspace/hyperhyperspace-core/commit/e7e00e4f596fb8813a30ac982df5ea660eec9caa