Implementing the CDUP command

CDUP is used to go up to the parent directory. In comparison to the CWD command implementation, it'll be a piece of cake! The CDUP command doesn't take arguments, so I'll let you add it to enums. Now, let's focus on the command implementation:

Command::CdUp => {
    if let Some(path) = self.cwd.parent().map(Path::to_path_buf) {
        self.cwd = path;
    }
    send_cmd(&mut self.stream, ResultCode::Ok, "Done");
}

And that's it. There's need to check if the parent folder exists, as it does. And if we're already at the root, then there's no need to do anything. Isn't it wonderful?