Implemented RenameFile and RenameDirectory in FS:USER

In other news, I propose an embargo on all merges until #147 is merged. That includes this PR. 
Kind of unrelated to this PR, but it might be nice to have something like `FileUtil::Path::Join(GetMountPoint(), dest_path.AsString())` to join paths in a platform-correct way, and insert the correct separator (instead of relying on `GetMountPoint` to have it already).