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/LibJS/Runtime/ScriptFunction.cpp

@@ -33,7 +33,7 @@
 
 namespace JS {
 
-ScriptFunction::ScriptFunction(const ScopeNode& body, Vector<FlyString> parameters)
+ScriptFunction::ScriptFunction(const Statement& body, Vector<FlyString> parameters)
     : m_body(body)
     , m_parameters(move(parameters))
 {