Use MonadFail (does not build under ghc 8.8)

[andreasabel]

The latest release (1.21.0) does not build under ghc 8.8.1, since it uses the old Monad.fail method (instead of MonadFail.fail):

[ 7 of 18] Compiling Language.Haskell.Exts.Fixity ( src/Language/Haskell/Exts/Fixity.hs, dist/build/Language/Haskell/Exts/Fixity.o )

src/Language/Haskell/Exts/Fixity.hs:72:25: error:
    • Could not deduce (MonadFail m) arising from a use of ‘infFix’
      from the context: Monad m
        bound by the type signature for:
                   applyFixities :: forall (m :: * -> *).
                                    Monad m =>
                                    [Fixity] -> Exp SrcSpanInfo -> m (Exp SrcSpanInfo)
        at src/Language/Haskell/Exts/Fixity.hs:72:3-15

[leonschoorl]

This was fixed by #440 and 1.21.1 is now on hackage