Stabilize hs-client APIs, mark feature as non-experimental.

Right now our onion service code is all/mostly behind features that are marked as experimental. We should go over these APIs and mark them as stable.

The following crates currently have experimental-marked onion-service APIs, or are themselves "inherently experimental":

• tor-error
• tor-hscrypto
• tor-proto
• tor-netdoc
• tor-netdir
• tor-circmgr
• tor-dirclient
• tor-keymgr
• tor-hsclient
• arti-client
• arti

(We shouldn't stabilize tor-hsservice or any of the hs-service features yet, since those aren't implemented.)