Provide option to detect modification based on hash, rather than on save location

[tefkah]

It would be nice if only the nodes that have been modified would get saved, rather than relying on the location of saving.

One way to do this would be to calculate the hash of the node-tree and compare that to a previous value.

The main problem with this approach is that the hash would obviously change every save, because the hash of cannot include itself (I'm pretty sure). So then I would need to compare hashes excluding the property drawer, which I don't really know how to do.