Fixtype of increment and decrement operators [C:6.5.3] [C:6.5.2] [C:A.2.1].
This is a tagged union type, introduced by fty::deftagsum.
These are the ++ and -- operators, for pre- and post- increment and decrement. They are already part of unop, but we also need a fixtype for just the two of them, so we can form lists in inc/dec-op-list.