optimization: store just IDs of child nodes

IDs are a lot smaller, hence quicker to manage.
Any time we want to refer back to the full node we look it up in its topology.

(This relies on nobody trying to store a rendered node as a child).

render/container.go

@@ -103,13 +103,7 @@ func FilterEmpty(topology string, r Renderer) Renderer {
 // topology.
 func HasChildren(topology string) FilterFunc {
 	return func(n report.Node) bool {
-		count := 0
-		n.Children.ForEach(func(child report.Node) {
-			if child.Topology == topology {
-				count++
-			}
-		})
-		return count > 0
+		return len(n.ChildIDs) > 0
 	}
 }