Use C++ filesystem library

[jgfoster]

"<filesystem> is technically available in C++11 and later. The implementation lives in the std::__fs::filesystem namespace..." Clang Developers.

To move from C++0x to C++11, see Arduino-CI/arduino_ci#146.

[jgfoster]

It proved too difficult to get the various pieces working so I just built an in-memory mock file system and our existing tests pass. See #11.