simplify sorting of setof items

That is a simplification which makes the implementation use libc's qsort and memcmp to sort elements of SET OF.

Checklist

• Code modified for feature
• Test suite updated with functionality tests

Reviewer's checklist:

• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent with other code
• This feature/change has adequate documentation added
• No obvious mistakes in the code