routing+refactor: let BlindedEdge carry BlindedPayment

This commit is purely a refactor. In it, we let the `BlindedEdge` struct
carry a pointer to the `BlindedPayment` that it was derived from. This
is done now because later on in the PR series, we will need more
information about the `BlindedPayment` that an edge was derived from.

Since we now pass in the whole BlindedPayment, we swap out the
`cipherText` member for a `hopIndex` member so that we dont carry around
two sources of truth in the same struct.

routing/router.go

@@ -2001,6 +2001,7 @@ func NewRouteRequest(source route.Vertex, target *route.Vertex,
 		// Assume that we're starting off with a regular payment.
 		requestHints  = routeHints
 		requestExpiry = finalExpiry
+		err           error
 	)
 
 	if blindedPayment != nil {
@@ -2038,7 +2039,10 @@ func NewRouteRequest(source route.Vertex, target *route.Vertex,
 			requestExpiry = blindedPayment.CltvExpiryDelta
 		}
 
-		requestHints = blindedPayment.toRouteHints()
+		requestHints, err = blindedPayment.toRouteHints()
+		if err != nil {
+			return nil, err
+		}
 	}
 
 	requestTarget, err := getTargetNode(target, blindedPayment)