Form definition (de)serialization

[vil1]

We need to be able to transfer and store for definitions.

This implies coming up with a serialization protocol for definitions, which can prove tricky because definitions contain functions at the leaves (and serializing functions is non trivial).

A simple approach would be to limit the validations (currently implemented as Kleisli[\/[String, ?], String, FormValue]) to a simple grammar (ADT).