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.
Needs some error handling instead of just aborting the emu on invalid paths, but this can be made as a follow-up PR after #147 is merged.