Add support for std::map<> comparator template argument for C#.

Allow exporting maps using non-default comparison function.

This seems to be already supported by some languages (e.g. Python) but not others. I only did it for C# right now but could/probably will do the same thing for Java later. However I can't do it for all the others, hence I also can't add the tests for the maps with non-default comparators to `Examples/test-suite/li_std_map.i`.