Previously, we'd use the value of nextChanID to infer whether a payload
was for the final hop in a route. This commit updates our packing logic
to explicitly signal to account for blinded routes, which allow zero
value nextChanID in intermediate hops. This is a preparatory commit
that allows us to more thoroughly validate payloads.
Previously, we were using nextChanID to determine whether a hop
payload is for the final recipient. This is no longer suitable in a
route-blinding world where intermediate hops are allowed to have zero
nextChanID TLVs (as this information is provided to forwarding nodes
in their encrypted data). This commit updates payload reading to use
the signal provided by sphinx that we are on the last packet, rather
than implying it from the contents of a hop.
hop.Payload and route.Hop are analogs, with onion payloads encoded from
route.Hops and decoded to hop.Payloads. For checking equality of
encoding/decoding, we implement a helper function to convert
hop.Payloads into route.Hops.
