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.

AK/HashMap.h

@@ -70,6 +70,19 @@ public:
 
     void dump() const { m_table.dump(); }
 
+    V get(const K& key) const
+    {
+        auto it = find(key);
+        if (it == end())
+            return V();
+        return (*it).value;
+    }
+
+    bool contains(const K& key) const
+    {
+        return find(key) != end();
+    }
+
 private:
     HashTable<Entry, EntryTraits> m_table;
 };