New method `as_string_view` for so_5::stats::prefix_t and so_5::stats::suffix_t

[eao197]

The so_5::stats::prefix_t and so_5::stats::suffix_t have only c_str() method. It's because this types were created in pre-C++17 era. But after switching to C++17 it's possible to add as_string_view method to these types. Something like that:

[[nodiscard]] std::string_view as_string_view() const;

[eao197]

It's a part of v.5.8.2 release.