Allow to have builtin item function to return a value

This does some refactoring to allow builtin item functions to return a
value:
 - builtin member functions are no longer BuiltinFunction, but they are
   just normal NamedReference
 - Move special case for them in the LLR/eval

internal/compiler/object_tree.rs

@@ -1699,6 +1699,7 @@ impl Element {
                 declared_pure: p.pure,
                 is_local_to_component: true,
                 is_in_direct_base: false,
+                builtin_function: None,
             },
         )
     }