FileSys: Updates backend code to use FileSys::Path and adds proper support for binary filenames.

Updates our backend file system code to use the new Path class defined in #183 for file/directory paths. This allows the same code to be agnostic of different file path types supported by 3DS. Also adds proper support for binary filenames. Fixes #187.
I still don't think this is correct. Are you sure that the archive itself is returned, and not a special pseudo-file? Probably a good candidate for verification when we have code exec. I'd leave a TODO.