--------------------------------
   Quantify over unbound type vars in RULE lhs's
	--------------------------------

-- We need to gather the type variables mentioned on the LHS so we can
-- quantify over them.  Example:
--   data T a = C
--
--   foo :: T a -> Int
--   foo C = 1
--
--   {-# RULES "myrule"  foo C = 1 #-}
--
-- After type checking the LHS becomes (foo a (C a))
-- and we do not want to zap the unbound tyvar 'a' to (), because
-- that limits the applicability of the rule.  Instead, we
-- want to quantify over it!

This commit fixes the problem, discovered by Manuel.  It uses a
free-variable finder for RULE lhs's (TcRule.ruleLhsTvs) which relies
on the fact that the LHS of a rule can only take ver forms
(c.f RnSource.validRuleLhs).