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/help.cpp b/tools/nfdc/help.cpp
index 052d16c..357a647 100644
--- a/tools/nfdc/help.cpp
+++ b/tools/nfdc/help.cpp
@@ -32,9 +32,7 @@
 #include <cstring>
 #include <unistd.h>
 
-namespace nfd {
-namespace tools {
-namespace nfdc {
+namespace nfd::tools::nfdc {
 
 NDN_LOG_INIT(nfdc.Help);
 
@@ -98,6 +96,4 @@
   }
 }
 
-} // namespace nfdc
-} // namespace tools
-} // namespace nfd
+} // namespace nfd::tools::nfdc