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.h

@@ -29,16 +29,20 @@
 #include <AK/ByteBuffer.h>
 #include <AK/RefCounted.h>
 
-class CNetworkResponse : public RefCounted<CNetworkResponse> {
+namespace Core {
+
+class NetworkResponse : public RefCounted<NetworkResponse> {
 public:
-    virtual ~CNetworkResponse();
+    virtual ~NetworkResponse();
 
     bool is_error() const { return m_error; }
     const ByteBuffer& payload() const { return m_payload; }
 
 protected:
-    explicit CNetworkResponse(ByteBuffer&&);
+    explicit NetworkResponse(ByteBuffer&&);
 
     bool m_error { false };
     ByteBuffer m_payload;
 };
+
+}