Add several methods that are not in this repo but in STL

[tatsy]

This PR introduces several methods that are not defined in this repo but defined in STL.

• exists() is defined as a method in filesystem namespace
  https://en.cppreference.com/w/cpp/filesystem/exists
• is_directory() is defined as a method in filesystem namespace
  https://en.cppreference.com/w/cpp/filesystem/is_directory
• string() is defined as a member of path object instead of str()
  https://en.cppreference.com/w/cpp/filesystem/path/string

For the backward compatibility, the methods, which are originally defined in this repo, are kept unchanged.