9.6 support

[wz1000]

This is based on the 9.6 branch of ghc exactprint: alanz/ghc-exactprint#123

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[guibou]

I think you meant that this is based on alanz/ghc-exactprint#124

I had to patch a bit in order to get this to build, using the following:

diff --git a/Retrie/Elaborate.hs b/Retrie/Elaborate.hs
index 1c88441..8be459f 100644
--- a/Retrie/Elaborate.hs
+++ b/Retrie/Elaborate.hs
@@ -103,7 +103,7 @@ allMatches _ [] = return []
 allMatches ctxt matchResults = do
   results <-
     forM matchResults $ \(sub, RewriterResult{..}) -> do
-      result <- lift $ liftIO $ rrTransformer ctxt $ MatchResult sub rrTemplate
+      result <- TransformT $ lift $ liftIO $ rrTransformer ctxt $ MatchResult sub rrTemplate
       return (rrQuantifiers, result)
   return
     [ project <$> result
diff --git a/Retrie/Rewrites/Function.hs b/Retrie/Rewrites/Function.hs
index 3944584..9f24977 100644
--- a/Retrie/Rewrites/Function.hs
+++ b/Retrie/Rewrites/Function.hs
@@ -53,7 +53,7 @@ dfnsToRewrites libdir specs am = fmap astA $ transformA am $ \ (L _ m) -> do
 getImports
   :: LibDir -> Direction -> Maybe (LocatedA ModuleName) -> TransformT IO AnnotatedImports
 getImports libdir RightToLeft (Just (L _ mn)) = -- See Note [fold only]
-  lift $ liftIO $ parseImports libdir ["import " ++ moduleNameString mn]
+  TransformT $ lift $ liftIO $ parseImports libdir ["import " ++ moduleNameString mn]
 getImports _ _ _ = return mempty
 
 matchToRewrites
diff --git a/Retrie/Types.hs b/Retrie/Types.hs
index 0f082b0..4bd7fb6 100644
--- a/Retrie/Types.hs
+++ b/Retrie/Types.hs
@@ -329,7 +329,7 @@ firstMatch _ [] = return NoMatch
 firstMatch ctxt ((sub, RewriterResult{..}):matchResults) = do
   -- 'firstMatch' is lazy in 'rrTransformer', only running it enough
   -- times to get the first valid MatchResult.
-  matchResult <- lift $ liftIO $ rrTransformer ctxt (MatchResult sub rrTemplate)
+  matchResult <- TransformT $ lift $ liftIO $ rrTransformer ctxt (MatchResult sub rrTemplate)
   case matchResult of
     MatchResult sub' _
       -- Check that all quantifiers from the original rewrite have mappings

This is because of the following change https://github.com/alanz/ghc-exactprint/pull/124/files#r1140532016