Annotations for structs, enums and variants

When dealing with some serialization libraries, it's required to add annotations to structs, enums and variants. There are currently three PR fixing this issue, could the maintainers review them and merge at least one of them?

#32 #33 #34
(the last one was submitted by me)

Any update for this. That would be great if we have annotations/attributes for structs/enums/variants