In SpecConstr.isValue, we recorded a ConVal for a big-lambda,
which seems wrong. I came across this when implementing System IF.
The code now reads:

  isValue env (Lam b e)
    | isTyVar b = case isValue env e of
  		    Just _  -> Just LambdaVal	-- NB!
  		    Nothing -> Nothing
    | otherwise = Just LambdaVal