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/aos/vision/image/image_dataset.cc b/aos/vision/image/image_dataset.cc
index a15ccb4..82a8886 100644
--- a/aos/vision/image/image_dataset.cc
+++ b/aos/vision/image/image_dataset.cc
@@ -4,8 +4,7 @@
 
 #include "aos/vision/image/image_types.h"
 
-namespace aos {
-namespace vision {
+namespace aos::vision {
 
 namespace {
 std::string GetFileContents(const std::string &filename) {
@@ -82,5 +81,4 @@
   return images;
 }
 
-}  // namespace vision
-}  // namespace aos
+}  // namespace aos::vision