cpp: upgrade to C++17 and use filesystem library instead

[wenchy]

Currently, C++ loader guarantees to support C++11 , so we can not use Filesystem library (since C++17).
In the future, when C++ loader upgrades to only support C++17 or above versions, then just uses Filesystem library (since C++17) instead.

References:

• https://en.cppreference.com/w/cpp/filesystem

Originally posted by @wenchy in #83 (comment)