Kernel: Move VM-related files into Kernel/VM/.

Also break MemoryManager.{cpp,h} into one file per class.

Kernel/VM/PhysicalPage.h

@@ -0,0 +1,46 @@
+#pragma once
+
+#include <Kernel/Assertions.h>
+#include <Kernel/types.h>
+#include <AK/Retained.h>
+
+class PhysicalPage {
+    friend class MemoryManager;
+    friend class PageDirectory;
+    friend class VMObject;
+public:
+    PhysicalAddress paddr() const { return m_paddr; }
+
+    void retain()
+    {
+        ASSERT(m_retain_count);
+        ++m_retain_count;
+    }
+
+    void release()
+    {
+        ASSERT(m_retain_count);
+        if (!--m_retain_count) {
+            if (m_may_return_to_freelist)
+                return_to_freelist();
+            else
+                delete this;
+        }
+    }
+
+    static Retained<PhysicalPage> create_eternal(PhysicalAddress, bool supervisor);
+    static Retained<PhysicalPage> create(PhysicalAddress, bool supervisor);
+
+    word retain_count() const { return m_retain_count; }
+
+private:
+    PhysicalPage(PhysicalAddress paddr, bool supervisor, bool may_return_to_freelist = true);
+    ~PhysicalPage() { }
+
+    void return_to_freelist();
+
+    word m_retain_count { 1 };
+    bool m_may_return_to_freelist { true };
+    bool m_supervisor { false };
+    PhysicalAddress m_paddr;
+};