Update list of koans

Author: LegalizeAdulthood

koans.md

@@ -2,5 +2,8 @@
 
 Note: These koans are not considered to be exhaustive.  Pull requests are welcome!
 
-- [iterator](iterator.cpp): Exploring the [<iterator>](http://en.cppreference.com/w/cpp/header/iterator) header.
-- [type_traits](type_traits.cpp): Exploring the [<type_traits>](http://en.cppreference.com/w/cpp/header/type_traits) header.
+- [iterator](iterator/iterator.cpp): Exploring the [<iterator>](http://en.cppreference.com/w/cpp/header/iterator) header.
+- [type_traits](type_traits/type_traits.cpp): Exploring the [<type_traits>](http://en.cppreference.com/w/cpp/header/type_traits) header.
+- [c_strings](c_strings/c_strings.cpp): Exploring C-style [string](http://en.cppreference.com/w/cpp/language/string_literal) and [character](http://en.cppreference.com/w/cpp/language/character_literal) literals.
+- [strings](strings/strings.cpp): Exploring [std::string](http://en.cppreference.com/w/cpp/string/basic_string).
+- [string_view](string_view/string_view.cpp): Exploring [std::string_view](http://en.cppreference.com/w/cpp/string/basic_string_view).