Follow-up from "tor-proto: Make start_conversation_last_hop() use a given hop, not the last."

The following discussion from !1469 (merged) should be addressed:

• @nickm started a discussion:

  Now that this is public, let's think a little more about the behavior we want from this type.

  We're using 0-indexing here, including in our Display implementation, but that gets a bit confusing for users when we call the third hop of a circuit "Hop 2". In various places we try to display hops as 1-indexed values, such "Extending circuit to hop {} with {:?}". But in other places we don't.

  I suggest that we add a ticket to rework impl Display from this type. I'm happy to do so if you think it's a good idea?