Implement linkspec manipulation for onion services

This will largely be the arti side of the followup from torspec#193 (closed).

added Next Onion Services Q1 labels

assigned to @nickm

I'd better reconstruct and summarize what I meant to do here before I forget it again.

• Implement code to enforce the restrictions of tor-spec § 5.1.2 on a list of link specifiers.
• Make a CircTarget that holds a bunch of linkspecs and outputs them in the order given.
• Make sure that the above CircTarget can get converted into an OwnedCircTarget with the right (lack of) cleanups and rules.
• Make sure that linkspec-sorting is done in the right places and not the wrong ones: We want to sort the linkspec lists we generate, but not the ones we get in an INTRODUCE message or a hsdesc.

added Q2 label and removed Q1 label

changed milestone to %Arti: Onion service support

changed time estimate to 24h

added Doing label and removed Next label

mentioned in merge request !1173 (closed)

added 6h of time spent

mentioned in issue #855 (closed)

Reducing the estimate here so I can split some into #855 (closed).

changed time estimate to 16h

added 3h of time spent

added Needs Review label and removed Doing label

mentioned in merge request !1177 (merged)

mentioned in commit ca96b77b

closed with merge request !1177 (merged)