Make `BasicStringRef` compatible with `std::experimental::string_view`

From [reddit](http://www.reddit.com/r/cpp/comments/2uvzns/release_of_c_format_10_a_small_safe_and_fast/codad52):

> `c_str` generally assumes a NUL-terminated string of characters, however the constructor `BasicStringRef(const Char *s, std::size_t size)` makes no such assumption.
