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.

DevTools/Inspector/RemoteProcess.h

@@ -58,6 +58,6 @@ private:
     pid_t m_pid { -1 };
     String m_process_name;
     NonnullRefPtr<RemoteObjectGraphModel> m_object_graph_model;
-    RefPtr<CLocalSocket> m_socket;
+    RefPtr<Core::LocalSocket> m_socket;
     NonnullOwnPtrVector<RemoteObject> m_roots;
 };