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/pathfind_test.go

@@ -3341,7 +3341,9 @@ func TestBlindedRouteConstruction(t *testing.T) {
 	// that make up the graph we'll give to route construction. The hints
 	// map is keyed by source node, so we can retrieve our blinded edges
 	// accordingly.
-	blindedEdges := blindedPayment.toRouteHints()
+	blindedEdges, err := blindedPayment.toRouteHints()
+	require.NoError(t, err)
+
 	carolDaveEdge := blindedEdges[carolVertex][0]
 	daveEveEdge := blindedEdges[daveBlindedVertex][0]