src/c-types-impl.lisp: Refactor pretty-printing of function types.

src/c-types-impl.lisp: Refactor pretty-printing of function types.

This will pay off later.  Even now, it's a bit easier to read.

If you pay attention, you'll notice that there's an extra logical block
around the argument list which doesn't seem to do anything very useful.
That too will pay off later.  Nothing else has actually changed yet.