Handle std::string_view in top-level spreadsort

The top-level `spreadsort` function has specific overloads targeting `std::string` and `std::wstring` specifically. With C++17 around the corner, these overloads could also accept `std::string_view` and `std::wstring_view` when they are available.

It notably helps `std::string_view` to be more of a "drop-in replacement for `std::string`" as much as possible.

By the way, is `string_spread_sort` also intended to sort collections of `std::vector` or `std::deque`, or is it not suited for these cases?