- re-establish and test some behavior from previous versions, that
if a load() or refresh() event changes history (which...why...but anyway)
the state of the object is the same; currently it seems that history
gets reset but on a refresh, the object still goes into session.dirty
- simplify what we store in partials
- reorganize how create_row_processor() communicates up to
instances(), using a named tuple it can assign to directly.  this way
we never have to worry about that structure changing anymore, though
we are still having it append (key, fn) which is kind of awkward.
- inline _populators() into instance(), it's a little verbose but
saves an fn call