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/constants.cc b/y2020/constants.cc
index e30ef0e..b5456b4 100644
--- a/y2020/constants.cc
+++ b/y2020/constants.cc
@@ -20,8 +20,7 @@
 #include "y2020/control_loops/superstructure/intake/integral_intake_plant.h"
 #include "y2020/control_loops/superstructure/turret/integral_turret_plant.h"
 
-namespace y2020 {
-namespace constants {
+namespace y2020::constants {
 
 const int Values::kZeroingSampleSize;
 
@@ -200,5 +199,4 @@
   return *values;
 }
 
-}  // namespace constants
-}  // namespace y2020
+}  // namespace y2020::constants