Function string2unsigned

example/utility/to_string.cpp:40–46

   }
   template <class Integer>
   Integer string2unsigned(const std::string& s)
   {  Integer result = 0;
      size_t index   = 0;
      while( index < s.size() )
         result = Integer(10 * result + Integer(s[index++]) - '0');
      return result;
   }
   template <class Integer>
   bool signed_integer(void)
   {  bool ok = true;