Data types for `send` and `receive` aren't preserved by `toJSON`

[mx00s]

As of 0398a0a the data types for send and receive operations are encoded in Dhall as Type, but there's no general function for converting Type -> Text.

Perhaps the type used for data in send and receive operations should be bound as a type parameter on ./SimpleSession/Type, e.g. \(dataType : Type), and the toJSON function should take an additional parameter of type dataType -> JSON/Type to help with the conversion.