Reduce the arti crate's API surface; improve semver documentation.

This branch combines a few small changes surrounding the public API of the arti crate.

• We make the DNS-proxy feature optional (#532 (closed))
• We make most of the APIs in arti experimental-only. (#530 (closed))
• We document better what it implies to "void [one's] semver warranty." (#522 (closed))

I'm open to making more of the APIs in arti public again, but I think we should discuss them individually if so.

Closes #532 (closed). Closes #530 (closed). Closes #522 (closed).