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.
archivename_type, archivename_size and archivename_ptr can be replaced with a FileSys::Path, exactly as you did for filename, here and in OpenArchive.