Expose arti_client::Result alias

On https://forum.zcashcommunity.com/t/arti-a-pure-rust-tor-implementation-for-zcash-and-beyond/38776/55 , teor suggests that we make our arti_client::Result type public.

There's no reason why not to do that as far as I can see. We can do it as part of #247 (closed), or independently.