Move next node ID to Resolver
This moves the `next_node_id` method(s) and related tracking information to the resolver. By doing so, we also remove the OneThread and Cell on next_node_id in Session in this move, which means that the new code is simpler and less "interesting" as it doesn't tie itself to a single thread.
This required moving some of the pretty-printing logic around, but this was just copying the code without any semantic changes, so it's just a second commit instead of a separate PR; I can polish it up a bit more if desired.
bors
86c28325ff