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/y2019/vision/image_writer.cc b/y2019/vision/image_writer.cc
index 4ca9b96..9b73a81 100644
--- a/y2019/vision/image_writer.cc
+++ b/y2019/vision/image_writer.cc
@@ -6,8 +6,7 @@
 
 #include "glog/logging.h"
 
-namespace y2019 {
-namespace vision {
+namespace y2019::vision {
 
 ImageWriter::ImageWriter() {
   LOG(INFO) << "Initializing image writer";
@@ -38,5 +37,4 @@
   }
 }
 
-}  // namespace vision
-}  // namespace y2019
+}  // namespace y2019::vision