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/motors/button_board.cc b/motors/button_board.cc
index 1d20fdb..e509ea6 100644
--- a/motors/button_board.cc
+++ b/motors/button_board.cc
@@ -16,8 +16,7 @@
 #include "motors/usb/usb.h"
 #include "motors/util.h"
 
-namespace frc971 {
-namespace motors {
+namespace frc971::motors {
 namespace {
 
 struct JoystickAdcReadings {
@@ -405,5 +404,4 @@
   return 0;
 }
 
-}  // namespace motors
-}  // namespace frc971
+}  // namespace frc971::motors