Refactor and modernize namespace declarations

 * Completely remove inline namespace v2
 * Flatten some unnecessarily deep namespace nesting
 * Move DummyClientFace, Segmenter, SegmentFetcher to namespace ndn
 * Move all unit tests to namespace ndn::tests

Change-Id: I8bcfcf9fd669936a3277d2d5d505f765b4b05742
diff --git a/tests/benchmarks/timed-execute.hpp b/tests/benchmarks/timed-execute.hpp
index b33873d..7ce5a82 100644
--- a/tests/benchmarks/timed-execute.hpp
+++ b/tests/benchmarks/timed-execute.hpp
@@ -24,8 +24,7 @@
 
 #include "ndn-cxx/util/time.hpp"
 
-namespace ndn {
-namespace tests {
+namespace ndn::tests {
 
 template<typename F>
 time::nanoseconds
@@ -37,7 +36,6 @@
   return after - before;
 }
 
-} // namespace tests
-} // namespace ndn
+} // namespace ndn::tests
 
 #endif // NDN_CXX_TESTS_BENCHMARKS_TIMED_EXECUTE_HPP