comparison operators: add support for `std::string` and `std::string_view`

[timblechmann]

static_string lacks the comparison operators to interact with std::string and std::string_view. this limits its usefulness when interacting with these types. e.g. when using a string_view to perform a lookup in a std::map<static_string, ...>

[timblechmann]

i've done some more experiments of going through boost::core::string_view. there are a few caveats, though:

• boost::core::string_view implicitly assumes std::char_traits<CharT>
• it starts becoming a can of worms when keeping static_strings::basic_string_view a boost::basic_string_view

it would certainly be a cleaner approach, though

[vinniefalco]

char traits is a design flaw in C++

[alandefreitas]

Obsoleted by #45