Postfix type operator support

Motivation

Just as there's support for postfix operators on the value level, it would be nice to have postfix operators on the type level:

data Some f where
  Some :: f a -> Some f

type (?) = Some

thing :: f ? -> f ?
thing (Some f) = Some f

Currently, GHC says "Operator applied to too few arguments: ?".

Proposal

Instead, support postfix operators on the type level, by parsing f ? as (?) f.