Make PageDirectory store physical pages in a HashMap.

This container is really just there to keep a retain on the individual
PhysicalPages for each page table. A HashMap does the job with far greater
space efficiency.

Kernel/MemoryManager.h

@@ -67,7 +67,7 @@ public:
 
 private:
     RetainPtr<PhysicalPage> m_directory_page;
-    RetainPtr<PhysicalPage> m_physical_pages[1024];
+    HashMap<unsigned, RetainPtr<PhysicalPage>> m_physical_pages;
 };
 
 class VMObject : public Retainable<VMObject> {