Remove prefix arrow notation for GADTs
Currently, GHC supports prefix (->)
arrows in a GADT declaration (#9096 (closed)):
data T a where
MkT :: (->) a (T a)
However, this special casing is brittle and Richard and I think we should remove it. It doesn't support (->) @k1 @k2 t1 t2
, type synonyms for (->)
nor type families. It's best to think of arrows ->
in a data constructor as separating fields.