Kernel: Make the x86 paging code slightly less insane.

Instead of PDE's and PTE's being weird wrappers around dword*, just have
MemoryManager::ensure_pte() return a PageDirectoryEntry&, which in turn has
a PageTableEntry* entries().

I've been trying to understand how things ended up this way, and I suspect
it was because I inadvertently invoked the PageDirectoryEntry copy ctor in
the original work on this, which must have made me very confused..

Anyways, now things are a bit saner and we can move forward towards a better
future, etc. :^)

Kernel/VM/PageDirectory.h

@@ -15,7 +15,7 @@ public:
     ~PageDirectory();
 
     dword cr3() const { return m_directory_page->paddr().get(); }
-    dword* entries() { return reinterpret_cast<dword*>(cr3()); }
+    PageDirectoryEntry* entries() { return reinterpret_cast<PageDirectoryEntry*>(cr3()); }
 
     void flush(VirtualAddress);