Removes .size() requirement from powerset argument
Removes .size() requirement from powerset argument

By just counting the elements when building combinators, there is no
need to call .size() on an unknown type Container

tempbottle/cppitertools,tempbottle/cppitertools,tempbottle/cppitertools,jkhoogland/cppitertools,ryanhaining/cppitertools,jkhoogland/cppitertools,jkhoogland/cppitertools,ryanhaining/cppitertools,ryanhaining/cppitertools