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/canonizer.cpp b/tools/nfdc/canonizer.cpp
index a44a2bf..c6f150b 100644
--- a/tools/nfdc/canonizer.cpp
+++ b/tools/nfdc/canonizer.cpp
@@ -25,9 +25,7 @@
 
 #include "canonizer.hpp"
 
-namespace nfd {
-namespace tools {
-namespace nfdc {
+namespace nfd::tools::nfdc {
 
 std::pair<std::optional<FaceUri>, std::string>
 canonize(ExecuteContext& ctx, const FaceUri& uri)
@@ -56,6 +54,4 @@
   return {FindFace::Code::CANONIZE_ERROR, msg};
 }
 
-} // namespace nfdc
-} // namespace tools
-} // namespace nfd
+} // namespace nfd::tools::nfdc