Removes parentheses around GADT constructors defined as operators

[jmcarthur]

Problem

The parentheses get removed, creating invalid syntax.

• [X] I checked the issue tracker for existing issues about this problem.
• [X] I'm using the latest version of hindent.

Input haskell

data FixedList n a where
  Nil :: FixedList 0 a
  (:::) :: a -> FixedList n a -> FixedList (n + 1) a

Expected output haskell

data FixedList n a where
  Nil :: FixedList 0 a
  (:::) :: a -> FixedList n a -> FixedList (n + 1) a

Actual output haskell

data FixedList n a where
  Nil :: FixedList 0 a
  ::: :: a -> FixedList n a -> FixedList (n + 1) a