LibCore: Put all classes in the Core namespace and remove the leading C

I've been wanting to do this for a long time. It's time we start being
consistent about how this stuff works.

The new convention is:

- "LibFoo" is a userspace library that provides the "Foo" namespace.

That's it :^) This was pretty tedious to convert and I didn't even
start on LibGUI yet. But it's coming up next.

Libraries/LibCore/CNetworkResponse.cpp

@@ -26,11 +26,15 @@
 
 #include <LibCore/CNetworkResponse.h>
 
-CNetworkResponse::CNetworkResponse(ByteBuffer&& payload)
+namespace Core {
+
+NetworkResponse::NetworkResponse(ByteBuffer&& payload)
     : m_payload(payload)
 {
 }
 
-CNetworkResponse::~CNetworkResponse()
+NetworkResponse::~NetworkResponse()
 {
 }
+
+}