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.