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/blinding.go

@@ -88,13 +88,13 @@ func (b *BlindedPayment) Validate() error {
 // the case of multiple blinded hops, CLTV delta is fully accounted for in the
 // hints (both for intermediate hops and the final_cltv_delta for the receiving
 // node).
-func (b *BlindedPayment) toRouteHints() RouteHints {
+func (b *BlindedPayment) toRouteHints() (RouteHints, error) {
 	// If we just have a single hop in our blinded route, it just contains
 	// an introduction node (this is a valid path according to the spec).
 	// Since we have the un-blinded node ID for the introduction node, we
 	// don't need to add any route hints.
 	if len(b.BlindedPath.BlindedHops) == 1 {
-		return nil
+		return nil, nil
 	}
 
 	hintCount := len(b.BlindedPath.BlindedHops) - 1
@@ -136,14 +136,13 @@ func (b *BlindedPayment) toRouteHints() RouteHints {
 		ToNodeFeatures: features,
 	}
 
-	hints[fromNode] = []AdditionalEdge{
-		&BlindedEdge{
-			policy:        edgePolicy,
-			cipherText:    b.BlindedPath.BlindedHops[0].CipherText,
-			blindingPoint: b.BlindedPath.BlindingPoint,
-		},
+	edge, err := NewBlindedEdge(edgePolicy, b, 0)
+	if err != nil {
+		return nil, err
 	}
 
+	hints[fromNode] = []AdditionalEdge{edge}
+
 	// Start at an offset of 1 because the first node in our blinded hops
 	// is the introduction node and terminate at the second-last node
 	// because we're dealing with hops as pairs.
@@ -169,14 +168,13 @@ func (b *BlindedPayment) toRouteHints() RouteHints {
 			ToNodeFeatures: features,
 		}
 
-		hints[fromNode] = []AdditionalEdge{
-			&BlindedEdge{
-				policy: edgePolicy,
-				cipherText: b.BlindedPath.BlindedHops[i].
-					CipherText,
-			},
+		edge, err := NewBlindedEdge(edgePolicy, b, i)
+		if err != nil {
+			return nil, err
 		}
+
+		hints[fromNode] = []AdditionalEdge{edge}
 	}
 
-	return hints
+	return hints, nil
 }