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/additional_edge_test.go

@@ -42,9 +42,13 @@ func TestIntermediatePayloadSize(t *testing.T) {
 			hop: route.Hop{
 				EncryptedData: []byte{12, 13},
 			},
-			edge: &BlindedEdge{
-				cipherText: []byte{12, 13},
-			},
+			edge: &BlindedEdge{blindedPayment: &BlindedPayment{
+				BlindedPath: &sphinx.BlindedPath{
+					BlindedHops: []*sphinx.BlindedHopInfo{
+						{CipherText: []byte{12, 13}},
+					},
+				},
+			}},
 		},
 		{
 			name: "Blinded edge - introduction point",
@@ -52,10 +56,14 @@ func TestIntermediatePayloadSize(t *testing.T) {
 				EncryptedData: []byte{12, 13},
 				BlindingPoint: blindedPoint,
 			},
-			edge: &BlindedEdge{
-				cipherText:    []byte{12, 13},
-				blindingPoint: blindedPoint,
-			},
+			edge: &BlindedEdge{blindedPayment: &BlindedPayment{
+				BlindedPath: &sphinx.BlindedPath{
+					BlindingPoint: blindedPoint,
+					BlindedHops: []*sphinx.BlindedHopInfo{
+						{CipherText: []byte{12, 13}},
+					},
+				},
+			}},
 		},
 	}