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/tools/nfdc/main.cpp b/tools/nfdc/main.cpp
index a9d9c6f..78da697 100644
--- a/tools/nfdc/main.cpp
+++ b/tools/nfdc/main.cpp
@@ -31,9 +31,7 @@
 #include <fstream>
 #include <iostream>
 
-namespace nfd {
-namespace tools {
-namespace nfdc {
+namespace nfd::tools::nfdc {
 
 static int
 main(int argc, char** argv)
@@ -184,9 +182,7 @@
   }
 }
 
-} // namespace nfdc
-} // namespace tools
-} // namespace nfd
+} // namespace nfd::tools::nfdc
 
 int
 main(int argc, char** argv)