Fix calculation of hardest_arg.

Fix calculation of hardest_arg.

The argument properties are ordered from easiest to hardest.  The
harder the arg, the more complicated that code that must be generated
to return it from getChildren() and/or getChildNodes().  The old
calculation routine was bogus, because it always set hardest_arg to
the hardness of the last argument.  Now use max() to always set it to
the hardness of the hardest argument.