Consider destructuring of field types

[bernhardmgruber]

LLAMA can only rearrange fields inside the record dimension, but considers the final types in the leaf-fields as atomic. We could consider changing that and allow LLAMA to even take apart user defined types. A good example for that would be std::optional<T>, which is basically a record consisting of a T and a bool. LLAMA could store those two pieces in different locations but always put them back together when loaded.