output-item: Collapse the inheritance hierarchy into a single struct.

output-item: Collapse the inheritance hierarchy into a single struct.

When I designed this inheritance hierarchy, what was really needed was a
bit of a mystery.  Now, it's pretty clear what is really going to be here,
and it simplifies and clarifies code a lot to just have a struct with an
embedded union.  It also removes, net, about 1000 lines of code.