Use nested namespace

Nested namespace is introduced in C++ 17. This saves nearly 50 lines.
This change also puts tests into a nested namespace, which saves
unnecessary using directives.

Signed-off-by: Nan Zhou <nanzhoumails@gmail.com>
Change-Id: I7c3e38588fd5c2cbd83ac13ee24327318e8c06a4
diff --git a/argument.hpp b/argument.hpp
index af5ac46..e67a877 100644
--- a/argument.hpp
+++ b/argument.hpp
@@ -5,11 +5,7 @@
 #include <map>
 #include <string>
 
-namespace phosphor
-{
-namespace certs
-{
-namespace util
+namespace phosphor::certs::util
 {
 
 /**
@@ -40,6 +36,4 @@
     static const char* optionstr;
 };
 
-} // namespace util
-} // namespace certs
-} // namespace phosphor
+} // namespace phosphor::certs::util