arti-hyper tls-api dependency pulls in ring via webpki

We're trying to make it easy to avoid ring, since its license isn't GPL-compatible—see #493 (closed). (We aren't GPL, but we want to be friendly to those who are.)

But the arti-hyper crate uses tls-api, which uses webpki, which uses ring.

This isn't a huge deal, since arti-hyper isn't mandatory by any means, but it would be neat to do something about it at some point.