corrects iteratoriterator traits
corrects iteratoriterator traits

previously all the iterator_traits were defined as per the outer
iterator instead of the inner one. This surfaced when compiling with
libc++ (as opposed to libstdc++) which doesn't tighter checks

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