add class similar to std::string_view (non owning string reference)

Class is basically:

class string_view {

  const char *string;

  size_t length;

};

It's intended to replace all naked char * and const std::string:: & in functions arguments.
string_view is better because of a clear semantics: non-owning string. When you see a member or variable const char *smth; you can't say whether it own its buffer or not. For argument you don't know whether ownership was transferred to function or not. Similar to return char * from function.

string_view makes code easier to read. Also it has begin(), end() member which allows it's data to be used in range-for loops.

https://en.cppreference.com/w/cpp/string/basic_string_view