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.
Since this is for debug, it would be better to display whether the string is UCS-2 or not.