WL#421: Change a way in which ENUM and SET values are compared in aggregate functions

At present, when aggragating values from ENUM and SET functions, those are 
compared based on the string values of tuples, instead of the integer value
of each instance.

To change this, it will be necessary to make separate functions for the update
of results from the functions of comparing two values.

I have already done some work on it in 4.0, but have saved and aborted changes, 
as they belong in 4.1 or 5.0.

This will require major changes in item_sum.cc with some methods being split into
two .