LibWeb: Add fast_is<T>() for some DOM and layout node subclasses

The generic is<T>() uses dynamic_cast which is fine in the majority
of cases, but when one of them shows up in profiles, we can make it
faster by answering the is-a question manually.

Userland/Libraries/LibWeb/Layout/Box.h

@@ -134,13 +134,7 @@ private:
     OwnPtr<StackingContext> m_stacking_context;
 };
 
-}
-
-namespace AK {
 template<>
-inline bool is<Web::Layout::Box>(const Web::Layout::Node& input)
-{
-    return input.is_box();
-}
+inline bool Node::fast_is<Box>() const { return is_box(); }
 
 }