Infix data declaration heads do not handle more than two parameters

[nibro]

The following program incorrectly fails to parse due to the p parameter:

{-# LANGUAGE TypeOperators #-}
module IllDataTypeDeclTest where
  
data (f :+: g) p = L

[nibro]

Fixing this requires changes to the way data headers are represented in the AST, i.e. a non-trivial fix that will also require a major version bump.