Refactor simplify_function et al to centralize argument simplification.

We were doing the recursive simplification of function/operator arguments
in half a dozen different places, with rather baroque logic to ensure it
didn't get done multiple times on some arguments.  This patch improves that
by postponing argument simplification until after we've dealt with named
parameters and added any needed default expressions.

Marti Raudsepp, somewhat hacked on by me