Generic api type fold: unify servant-foreign and servant-docs

[phadej]

it seems that servant-foreign and servant-docs are moving into same direction
of collecting a lot of information about api type. I wonder if they can be unified in the longer run.
They are simple "folds" as the esult type is independent of endpoints (not true with servant-client e.g.), a bit how Foldable and Traversable are related.

I wonder if we can provide single but extensible GADT? to give common parts of servant-foreign and servant-docs.

[alpmestan]

Wouldn't handling of external non-standard combinators become a bit of a pain if we are to offer a simple solution for collecting data on all standard combinators?