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/vision/sift/sift971.cc b/y2020/vision/sift/sift971.cc
index afab9b4..21f9a97 100644
--- a/y2020/vision/sift/sift971.cc
+++ b/y2020/vision/sift/sift971.cc
@@ -118,8 +118,7 @@
 
 using namespace cv;
 
-namespace frc971 {
-namespace vision {
+namespace frc971::vision {
 namespace {
 
 #define USE_AVX2 0
@@ -1288,5 +1287,4 @@
   }
 }
 
-}  // namespace vision
-}  // namespace frc971
+}  // namespace frc971::vision