Need a way to get KH information from intro-point circuits.

In rend-spec-v3, we use the KH data from introduction points to bind ESTABLISH_INTRO messages to a specific circuit hop. We need a way to extract that information from a circuit.

Perhaps in the long run it would be better if instead of extracting this, we had a way to ask the circuit to give us a MAC object.

In most cases, we know whether we will ever need this information when the circuit is created.