CString Iterator & ZipLongest iterator

This release, two new iterators have been released:

• Zip longest, used to iterate over multiple containers, and stops when the longest container is at its end. If C++17, it uses std::optional, otherwise, it uses an internal optional, with the same syntax & basic functions as std::optional, so breaking changes are prevented when upgrading to C++17.
• C String, used to iterate over a C style string, without having to know its length.

Also a few functions were missing documentation which is now fixed.