Refactor and modernize namespace declarations

Move all unit tests to namespace nfd::tests

Delete unused header core/algorithm.hpp

Change-Id: I5591f0c5f3bb5db67f8b45fae95471f8a555ca68
diff --git a/daemon/table/pit-entry.cpp b/daemon/table/pit-entry.cpp
index e2e194f..b302c28 100644
--- a/daemon/table/pit-entry.cpp
+++ b/daemon/table/pit-entry.cpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /*
- * Copyright (c) 2014-2019,  Regents of the University of California,
+ * Copyright (c) 2014-2022,  Regents of the University of California,
  *                           Arizona Board of Regents,
  *                           Colorado State University,
  *                           University Pierre & Marie Curie, Sorbonne University,
@@ -27,8 +27,7 @@
 
 #include <algorithm>
 
-namespace nfd {
-namespace pit {
+namespace nfd::pit {
 
 Entry::Entry(const Interest& interest)
   : m_interest(interest.shared_from_this())
@@ -120,5 +119,4 @@
   }
 }
 
-} // namespace pit
-} // namespace nfd
+} // namespace nfd::pit