Nest all namespaces

The compiler supports this now.  We can type less going forward.
No functional changes.

Signed-off-by: Stephan Pleines <pleines.stephan@gmail.com>
Change-Id: I29d6fa4f9aacc0e381f1a7637294db0392466995
diff --git a/y2023/vision/yolov5.cc b/y2023/vision/yolov5.cc
index ba2a9f7..e62b106 100644
--- a/y2023/vision/yolov5.cc
+++ b/y2023/vision/yolov5.cc
@@ -30,8 +30,7 @@
 
 DEFINE_bool(visualize_detections, false, "Display inference output");
 
-namespace y2023 {
-namespace vision {
+namespace y2023::vision {
 
 class YOLOV5Impl : public YOLOV5 {
  public:
@@ -303,5 +302,4 @@
   return filtered_detections;
 };
 
-}  // namespace vision
-}  // namespace y2023
+}  // namespace y2023::vision