Support lenses for fields of higher rank types with GHC >= 9.2

[arybczak]

With https://gitlab.haskell.org/ghc/ghc/-/merge_requests/3220 merged (:tada: :exclamation: :tada:), the following compiles with GHC 9.1.0_20200927:

data Rank2 = Rank2 { rank2_x :: forall a. a -> a }

rank2_x_lens :: Lens' Rank2 (forall a. a -> a)
rank2_x_lens = lensVL $ \f (Rank2 x) -> (\x' -> Rank2 x') <$> f x

instance
  ( a ~ (forall x. x -> x)
  , b ~ (forall x. x -> x)
  ) => LabelOptic "x" A_Lens Rank2 Rank2 a b where
  labelOptic = rank2_x_lens

There are some unification issues that prevent convenient usage without intermediate type signatures (https://gitlab.haskell.org/ghc/ghc/-/issues/18759), but once they're ironed out, we'll be able to support all fields via labels :)