In this commit, we extend the Hop struct to carry an arbitrary set of
TLV values, and add a new field that allows us to distinguish between
the modern and legacy TLV payload.
We add a new `PackPayload` method that will be used to encode the
combined required routing TLV fields along any set of TLV fields that
were specified as part of path finding.
Finally, the `ToSphinxPath` has been extended to be able to recognize if
a hop needs the modern, or legacy payload.
In this commit, we add two new method so the `Record` struct: Type() and
Encode(). These are useful when a caller is handling a record and may
not know its underlying type and may need to encode a record in
isolation.
