brittany 0.12 doesn't recognised strictness annotations in GADTs

[infinity0]

$ cat Test.hs

{-# LANGUAGE GADTs #-}
data X where
  X :: !Int -> X

main :: IO ()
main = pure ()

$ brittany Test.hs
ERROR: brittany pretty printer returned syntactically invalid result.
70
$ brittany --version
brittany version 0.12.0.0
Copyright (C) 2016-2018 Lennart Spitzner
There is NO WARRANTY, to the extent permitted by law.

[lspitzner]

This very likely is the same underlying issue as #242, so

• this comes down to a bug in ghc-exactprint (Roundtripping breaks when using exactPrint on single GADT-with-forall alanz/ghc-exactprint#79).
• Does not appear wtih ghc<8.6.

(We really want our own GADT layouting to avoid this ..)