AK: Stop allowing implicit downcast with RefPtr and NonnullRefPtr

We were allowing this dangerous kind of thing:

RefPtr<Base> base;
RefPtr<Derived> derived = base;

This patch changes the {Nonnull,}RefPtr constructors so this is no
longer possible.

To downcast one of these pointers, there is now static_ptr_cast<T>:

RefPtr<Derived> derived = static_ptr_cast<Derived>(base);

Fixing this exposed a ton of cowboy-downcasts in various places,
which we're now forced to fix. :^)

Libraries/LibWeb/DOM/Document.cpp

@@ -221,7 +221,7 @@ void Document::layout()
 
     if (!m_layout_root) {
         LayoutTreeBuilder tree_builder;
-        m_layout_root = tree_builder.build(*this);
+        m_layout_root = static_ptr_cast<LayoutDocument>(tree_builder.build(*this));
     }
     m_layout_root->layout();
     m_layout_root->set_needs_display();