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/y2020/control_loops/drivetrain/localizer.cc b/y2020/control_loops/drivetrain/localizer.cc
index 8f8bf87..fabbb8e 100644
--- a/y2020/control_loops/drivetrain/localizer.cc
+++ b/y2020/control_loops/drivetrain/localizer.cc
@@ -6,9 +6,7 @@
             "If true, send LocalizerDebug messages on every tick, even if "
             "they would be empty.");
 
-namespace y2020 {
-namespace control_loops {
-namespace drivetrain {
+namespace y2020::control_loops::drivetrain {
 
 namespace {
 // Converts a flatbuffer TransformationMatrix to an Eigen matrix. Technically,
@@ -499,6 +497,4 @@
   return Zhat;
 }
 
-}  // namespace drivetrain
-}  // namespace control_loops
-}  // namespace y2020
+}  // namespace y2020::control_loops::drivetrain