LibCore: Remove all remaining C prefix references

LibCore's GZip is also moved into the Core namespace with this change.

Libraries/LibGUI/Application.cpp

@@ -69,7 +69,7 @@ int Application::exec()
 {
     int exit_code = m_event_loop->exec();
     // NOTE: Maybe it would be cool to return instead of exit()?
-    //       This would require cleaning up all the CObjects on the heap.
+    //       This would require cleaning up all the Core::Objects on the heap.
     exit(exit_code);
     return exit_code;
 }