These implementations are not optimized, i.e. are done in a naive way in
C#, rather than using C++ functions more efficiently, but are better
than nothing.
Create new Lib/csharp/std_set.i based on the existing std_map.i and run
li_std_set unit test for C# as well.
Notice that the set operations defined by the base ISet<> interface are
not implemented yet.