Allow free variables in reifyInstances

When I say

{-# LANGUAGE TemplateHaskell #-}

module Bug where

import Language.Haskell.TH

$( do insts <- reifyInstances ''Eq [ListT `AppT` VarT (mkName "a")]
      runIO $ putStrLn $ pprint insts
      return [] )

I get

Bug.hs:7:4:
    Not in scope: type variable ‘a’
    In the argument of reifyInstances: GHC.Classes.Eq [a]

But, I wanted the declaration for instance Eq a => Eq [a]. The error message isn't wrong, exactly -- a really isn't in scope -- but I think we can do better by just treating variables as fresh.

Trac metadata

Trac field	Value
Version	7.8.2
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Template Haskell
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