Kernel: Move PhysicalPage classes out of the heap into an array

By moving the PhysicalPage classes out of the kernel heap into a static
array, one for each physical page, we can avoid the added overhead and
easily find them by indexing into an array.

This also wraps the PhysicalPage into a PhysicalPageEntry, which allows
us to re-use each slot with information where to find the next free
page.

Kernel/VM/PageDirectory.h

@@ -32,6 +32,8 @@ public:
 
     ~PageDirectory();
 
+    void allocate_kernel_directory();
+
     FlatPtr cr3() const
     {
 #if ARCH(X86_64)