Provide a way to "finalize" deserializaton

[RReverser]

Sometimes, you can describe the structure almost perfectly in struct with just derive(Deserialize), but just need some extra steps to "finalize" it depending on some fields etc..

When you are deserializing struct directly, it's possible to just add extra method for that, but when it's part of other derive(Deserialize) items, it becomes less convenient, and you have to reimplement Deserialize completely or write deserialize_with everywhere.

It would be nice if serde would provide such hook for "finalization" in the core.

[dtolnay]

Thanks! We are tracking this in #642.

[RReverser]

Oh, sorry, missed it.