Kernel: Switch singletons to use new Singleton class

MemoryManager cannot use the Singleton class because
MemoryManager::initialize is called before the global constructors
are run. That caused the Singleton to be re-initialized, causing
it to create another MemoryManager instance.

Fixes #3226

Kernel/Devices/KeyboardDevice.h

@@ -45,6 +45,7 @@ class KeyboardDevice final : public IRQHandler
 public:
     using Event = KeyEvent;
 
+    static void initialize();
     static KeyboardDevice& the();
 
     virtual ~KeyboardDevice() override;