Flatten OnionHopData struct with the Realm0 struct.

Flatten OnionHopData struct with the Realm0 struct.

Previously OnionHopData contained a OnionRealm0HopData field however
instead of bumping the realm number, it has been replaced with a
length, used to indicte the length of a TLV-formatted object.

Because a TLV-formatted hop data can contain the same information as
a realm-0 hop data, we flatten the field and simply keep track of
what format it was in.