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/y2016/vision/stereo_geometry.cc b/y2016/vision/stereo_geometry.cc
index a58675c..673834c 100644
--- a/y2016/vision/stereo_geometry.cc
+++ b/y2016/vision/stereo_geometry.cc
@@ -1,7 +1,6 @@
 #include "y2016/vision/stereo_geometry.h"
 
-namespace y2016 {
-namespace vision {
+namespace y2016::vision {
 
 Calibration FindCalibrationForRobotOrDie(
     const ::std::string &robot_name, const CalibrationFile &calibration_file) {
@@ -14,5 +13,4 @@
           calibration_file.ShortDebugString().c_str());
 }
 
-}  // namespace vision
-}  // namespace y2016
+}  // namespace y2016::vision