Kernel: Introduce the DeviceManagement singleton

This singleton simplifies many aspects that we struggled with before:
1. There's no need to make derived classes of Device expose the
constructor as public anymore. The singleton is a friend of them, so he
can call the constructor. This solves the issue with try_create_device
helper neatly, hopefully for good.
2. Getting a reference of the NullDevice is now being done from this
singleton, which means that NullDevice no longer needs to use its own
singleton, and we can apply the try_create_device helper on it too :)
3. We can now defer registration completely after the Device constructor
which means the Device constructor is merely assigning the major and
minor numbers of the Device, and the try_create_device helper ensures it
calls the after_inserting method immediately after construction. This
creates a great opportunity to make registration more OOM-safe.

Kernel/Devices/DeviceManagement.h

@@ -0,0 +1,54 @@
+/*
+ * Copyright (c) 2021, Liav A. <liavalb@hotmail.co.il>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Badge.h>
+#include <AK/NonnullRefPtrVector.h>
+#include <AK/OwnPtr.h>
+#include <AK/RefPtr.h>
+#include <AK/Time.h>
+#include <AK/Types.h>
+#include <Kernel/API/KResult.h>
+#include <Kernel/API/TimePage.h>
+#include <Kernel/Arch/x86/RegisterState.h>
+#include <Kernel/Devices/Device.h>
+#include <Kernel/Devices/NullDevice.h>
+#include <Kernel/UnixTypes.h>
+
+namespace Kernel {
+
+class DeviceManagement {
+    AK_MAKE_ETERNAL;
+
+public:
+    DeviceManagement();
+    static void initialize();
+    static DeviceManagement& the();
+    void attach_null_device(NullDevice const&);
+
+    void after_inserting_device(Badge<Device>, Device&);
+    void before_device_removal(Badge<Device>, Device&);
+
+    void for_each(Function<void(Device&)>);
+    Device* get_device(unsigned major, unsigned minor);
+    NullDevice const& null_device() const;
+    NullDevice& null_device();
+
+    template<typename DeviceType, typename... Args>
+    static inline KResultOr<NonnullRefPtr<DeviceType>> try_create_device(Args&&... args)
+    {
+        auto device = TRY(adopt_nonnull_ref_or_enomem(new DeviceType(forward<Args>(args)...)));
+        device->after_inserting();
+        return device;
+    }
+
+private:
+    RefPtr<NullDevice> m_null_device;
+    MutexProtected<HashMap<u32, Device*>> m_devices;
+};
+
+}