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/y2018/constants.cc b/y2018/constants.cc
index 189ea5b..41c8f3a 100644
--- a/y2018/constants.cc
+++ b/y2018/constants.cc
@@ -20,8 +20,7 @@
 #define M_PI 3.14159265358979323846
 #endif
 
-namespace y2018 {
-namespace constants {
+namespace y2018::constants {
 namespace {
 
 const uint16_t kCompTeamNumber = 971;
@@ -163,5 +162,4 @@
   return *values[team_number];
 }
 
-}  // namespace constants
-}  // namespace y2018
+}  // namespace y2018::constants