optimisation: make Memoise(Memoise(...)) only memoise once

All outputs of Memoise() are fixpoints to the function, i.e. feeding
them as inputs to the function just returns them.

We don't hit this optimisation in current code but have had instances
in the past.

render/memoise.go

@@ -27,6 +27,9 @@ type memoise struct {
 
 // Memoise wraps the renderer in a loving embrace of caching.
 func Memoise(r Renderer) Renderer {
+	if _, ok := r.(*memoise); ok {
+		return r // fixpoint
+	}
 	return &memoise{
 		Renderer: r,
 		id:       fmt.Sprintf("%x", rand.Int63()),

render/memoise_test.go

@@ -20,6 +20,11 @@ func TestMemoise(t *testing.T) {
 		return render.Nodes{Nodes: report.Nodes{rpt.ID: report.MakeNode(rpt.ID)}}
 	})
 	m := render.Memoise(r)
+
+	if render.Memoise(m) != m {
+		t.Errorf("Memoised renderers should be fixpoints.")
+	}
+
 	rpt1 := report.MakeReport()
 
 	result1 := m.Render(rpt1)