Revert to using f instead of 0 to indicate final form.  Using 0 is not as good
because if for some reason a reduction function did return f, the reference
counting in eval() would be messed up because it depends on no aliasing.  But
if we use f to indicate a final form, that has a valid meaning.  So now we've
ended that brief experiment with using 0.

While we're at it, I renamed standard_name to type_standard_name.  Now we can
consistently say that all reduction functions either start with "fexl_" (if
it's callable) or "type_" (if it's internal).