Drop support for parentheses in GADT constructors
See user manual 22.214.171.124. Formal syntax for GADTs
Vlad is suggesting that we do not support parens, thus striking out the production
prefix_gadt_body ::= '(' prefix_gadt_body ')'
So you can't write
data T where T1 :: (Int -> T) T2 :: (forall a. a -> T) T3 :: forall a. (a -> T) T4 :: forall a. (a -> (T))
GHC currently allows all four, simply discarding the parens entirely, very early in parsing, specifically in