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/control_loops/superstructure/arm/arm.cc b/y2023/control_loops/superstructure/arm/arm.cc
index 7144e57..51efc55 100644
--- a/y2023/control_loops/superstructure/arm/arm.cc
+++ b/y2023/control_loops/superstructure/arm/arm.cc
@@ -3,10 +3,7 @@
 #include "y2023/control_loops/superstructure/roll/integral_hybrid_roll_plant.h"
 #include "y2023/control_loops/superstructure/roll/integral_roll_plant.h"
 
-namespace y2023 {
-namespace control_loops {
-namespace superstructure {
-namespace arm {
+namespace y2023::control_loops::superstructure::arm {
 namespace {
 
 namespace chrono = ::std::chrono;
@@ -359,7 +356,4 @@
   return status_builder.Finish();
 }
 
-}  // namespace arm
-}  // namespace superstructure
-}  // namespace control_loops
-}  // namespace y2023
+}  // namespace y2023::control_loops::superstructure::arm