Add necessary methods to support finding nodes/versions resulting from a split

Can be used in a transactional system to update the absent set after a write-my-reads insert that results in a split.

I can squash the commits to make the git log nicer?
So, the idea is that oldv_/newv_/oldn_ all correspond to the same node, namely, the "original" node before any splits occur. So the idea (hopefully, there could definitely be a bug somewhere here) is that at this point we're about to unlock our original node and start working with the new trie layer farther down.

And then the new trie layer we're about to work with (which we'll set to n_ a few lines below) will get added to newnodes_ later on.