Fortran filesystem

For performance & robustness, use C++17 std::string_view for C++ inputs generally. This is generally better than const std::string&.

add set_cwd() function which is just like chdir()

Make get_homedir, used by expanduser, canonical, etc. use backup methods if env var HOME or USERPROFILE isn't set.

as_posix() isn't implicitly used by canonical and similar anymore. The user is responsible to use appropriate escaping and file separators.