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

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`.