---------------------------------------------------
	Important fix to the handling of class methods that
	      mention their own class type variable
	---------------------------------------------------

[NB: I'm not 100% certain that this commit is independent of the
     Template-Haskell-related commit I'm doing at the same time.
     I've tried to separate them but may not have succeeded totally.]

This bug gives utterly bogus (detected by Core Lint) programs.
Isaac Jones discovered it.  Here's an example, now enshrined as tc165.

    class C a where
	f :: (Eq a) => a

    instance C () where
	f = f

The instance decl was translated as

    dfC() = MkC (let f = \dEq -> f in f)

which is utterly wrong.  Reason: the 'f' on the left was being treated
as an available Inst, but it doesn't obey INVARIANT 2 for Insts, which
is that they are applied to all their dictionaries.  (See the data type
decl for Inst.)

Solution: don't include such class methods in the available Insts.