Deprecate generic encoding

[Chuckame]

Encoding to generic data and decoding from generic data is quite complex to maintain as it replicates anything from the direct encoding, plus some specificities. It is more a converter than a real serialization.

But, it is still needed as some common libraries are using generic records and so on (like #218).

Let's already deprecate this part to enforce users to notify their specific needs.

Final goal: simplify the library, while keeping good performances.