Add support for struct variants in untagged and adjacently tagged enu…

…ms in non self-describing formats

Visitor that passed th the deserialize_any supports only visit_map method,
so we can always requests deserialize_map

serde_derive/src/de.rs

@@ -1051,7 +1051,7 @@ fn deserialize_struct(
             _serde::Deserializer::deserialize_map(#deserializer, #visitor_expr)
         },
         StructForm::Untagged(_, deserializer) => quote! {
-            _serde::Deserializer::deserialize_any(#deserializer, #visitor_expr)
+            _serde::Deserializer::deserialize_map(#deserializer, #visitor_expr)
         },
     };