This patch does two related things

* Combines the occurrence-check logic in the on-the-fly unifier with
  that in the constraint solver.  They are both doing the same job,
  after all.  The resulting code is now in TcUnify:
     metaTyVarUpdateOK
     occCheckExpand
     occCheckForErrors (called in TcErrors)

* In doing this I disovered checking for family-free-ness and foralls
  can be unnecessarily inefficient, because it expands type synonyms.
  It's easy just to cache this info in the type syononym TyCon, which
  I am now doing.