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/print/usb.cc b/motors/print/usb.cc
index 9284ecd..45734b5 100644
--- a/motors/print/usb.cc
+++ b/motors/print/usb.cc
@@ -4,8 +4,7 @@
 
 #include "motors/core/kinetis.h"
 
-namespace frc971 {
-namespace motors {
+namespace frc971::motors {
 namespace {
 
 ::std::atomic<teensy::AcmTty *> global_stdout{nullptr};
@@ -62,5 +61,4 @@
   global_stdout.store(&stdout_tty_, ::std::memory_order_release);
 }
 
-}  // namespace motors
-}  // namespace frc971
+}  // namespace frc971::motors