Revised MemberExtractorComparer - replaced std comparers with stingraykit ones. Also optimized MemberExtractor - there's no need to copy member if we have pointer to it
Revised MemberExtractorComparer - replaced std comparers with stingraykit ones. Also optimized MemberExtractor - there's no need to copy member if we have pointer to it

GSGroup/stingraykit,GSGroup/stingraykit