adds constexpr to count()
adds constexpr to count()

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