More simplifications to the constraint solver

* inert_solved becomes dictionaries-only, inert_solved_dicts

* inert_solved_dicts is used only to cache the result of uses
  of a top level instance declaration, just like inert_solved_funeqs

* That in turn simplifies xCtFlavor and rewriteCtFlavor, because
  they no longer need a "should I cache" parameter.  (Moreover the
  settings for this parameter were very subtle; it's easy to get
  loops if you cache too much.  Caching only top-level instance
  uses is much safer, and eliminates all these subtle cases.)