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.
This could check for `path` being a 0x00-filled binary path before returning. I think that's what games try to pass to get the raw fs, right?