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/face/face.cpp b/daemon/face/face.cpp
index cc606d0..7b15dc5 100644
--- a/daemon/face/face.cpp
+++ b/daemon/face/face.cpp
@@ -25,8 +25,7 @@
 
 #include "face.hpp"
 
-namespace nfd {
-namespace face {
+namespace nfd::face {
 
 Face::Face(unique_ptr<LinkService> service, unique_ptr<Transport> transport)
   : afterReceiveInterest(service->afterReceiveInterest)
@@ -51,5 +50,4 @@
   return os;
 }
 
-} // namespace face
-} // namespace nfd
+} // namespace nfd::face