Refactor shared_str

[Xottab-DUTY]

1. std::string/std::string_view like interface
   1. Get rid of operator*
   2. Prohibit direct comparison with nullptr (via providing std::nullptr_t overloads for operator== and xr_strcmp)
   3. Add functions from std::string/std::string_view
      1. Treat it as a view of immutable string and add functions accordingly reasonable
      2. Already existing printf is acceptable, but we should decide if we should leave or not
2. Replace pointer with 32-bit identifier which will allow us to reduce the class size on 64-bit platforms
3. Computational requirements:
   • O(1) – access the underlying string
   • O(1) – search
   • Insertion/removal don't have strict requirements, but just don't make it O(N^2).
     O(log N) is very good, O(N) is acceptable

References, useful links:

• https://github.com/Tessil/array-hash