When we spot an equality arising from a functional dependency,
we now use that equality (a "wanted") to rewrite the work-item
constraint right away.  This avoids two dangers

 Danger 1: If we send the original constraint on down the pipeline
           it may react with an instance declaration, and in delicate
	   situations (when a Given overlaps with an instance) that
	   may produce new insoluble goals: see Trac #4952

 Danger 2: If we don't rewrite the constraint, it may re-react
           with the same thing later, and produce the same equality
           again --> termination worries.

To achieve this required some refactoring of FunDeps.lhs (nicer
now!).  

This patch also contains a couple of unrelated improvements

* A bad bug in TcSMonad.nestImplicTcS whereby the Tcs tyvars
  of an outer implication were not untouchable inside

* Improved logging machinery for the type constraint solver;
  use -ddump-cs-trace (probably with a wider default line width
  -dppr-cols=200 or something)