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/unit/util/io.t.cpp b/tests/unit/util/io.t.cpp
index 9658d04..934648a 100644
--- a/tests/unit/util/io.t.cpp
+++ b/tests/unit/util/io.t.cpp
@@ -27,8 +27,7 @@
 #include <boost/filesystem.hpp>
 #include <boost/mpl/vector.hpp>
 
-namespace ndn {
-namespace tests {
+namespace ndn::tests {
 
 BOOST_AUTO_TEST_SUITE(Util)
 BOOST_AUTO_TEST_SUITE(TestIo)
@@ -398,5 +397,4 @@
 BOOST_AUTO_TEST_SUITE_END() // TestIo
 BOOST_AUTO_TEST_SUITE_END() // Util
 
-} // namespace tests
-} // namespace ndn
+} // namespace ndn::tests