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/CHttpResponse.cpp

@@ -26,13 +26,17 @@
 
 #include <LibCore/CHttpResponse.h>
 
-CHttpResponse::CHttpResponse(int code, HashMap<String, String>&& headers, ByteBuffer&& payload)
-    : CNetworkResponse(move(payload))
+namespace Core {
+
+HttpResponse::HttpResponse(int code, HashMap<String, String>&& headers, ByteBuffer&& payload)
+    : NetworkResponse(move(payload))
     , m_code(code)
     , m_headers(move(headers))
 {
 }
 
-CHttpResponse::~CHttpResponse()
+HttpResponse::~HttpResponse()
 {
 }
+
+}