Full implementation of the filesystem submodule across windows, linux and OS X

• Implemented cross-platform permissions as per the C++ 17 standard. POSIX style permissions get properly mapped into Windows ACL.
• Added masx/flag manipulation helpers to the common submodule
• Cleanup of the code and the test suite
• Build passes on all platforms in CI

TODO:

• Additional test cases
• fine grained permissions tweaking on windows for sumbolic links