Fortran Pathlib

• create API.md document, to avoid huge README
• correct is_dir for windows GCC, to be correct
• add is_symlink for POSIX-only for now

Except for the new is_symlink(), all of fortran-pathlib is OS-agnostic and also completely works with Intel/GCC (or other future Fortran 2008 compilers). For is_symlink(), extra work would be needed for Windows to handle symlinks.

• name surrogate reparse point IO_REPARSE_TAG_SYMLINK
• directory junction IO_REPARSE_TAG_MOUNT_POINT

C stdlib lstat() does not exist on Windows even with MinGW. C++ Boost and Python os.stat have implemented symlinks on Windows.