Marking this one as resolved as it's not something we're planning to do and
there are many many things that "could be tried". That doesn't mean this isn't a
good idea -- it could have a quite nice positive impact on memory use if we
could implement it well. But I don't think it meshes well with the new state
registry approach.

As far as I know, the eager open lists currently store complete states. If
memory is more limiting than time, it might be preferrable to store
predecessor/action pairs, as in the lazy search. Experiments needed.