This patch arranges that derived instances use the same instance-decl
checking code as user-defined instances.  That gives greater consistency
in error messages.

Furthermore, the error description if this consistency check fails is now
much more explicit.  For example, drvfail003 now says
     Variable occurs more often in a constraint than in the instance head
       in the constraint: Show (v (v a))
     (Use -fallow-undecidable-instances to permit this)
     In the derived instance
       instance (Show (v (v a))) => Show (Square_ v w a)