optional serde integration

[kitlith]

prompted by https://rust-lang.github.io/api-guidelines/interoperability.html#c-serde

need to investigate how this would work with serde.

[kitlith]

The easiest structure output would essentially be a literal encoding of the cons list. I don't think this is a nice solution, though.

It would make more sense to essentially "look like a struct", but that leaves the issue of providing field names.

A sketch:

trait FieldName {
    const NAME: &'static str;
}

trait Serializable { /* ... */}

impl Serializable for TyEnd { /* ... */ }

impl<T, R> Serializable for Ty<T, R> where T: FieldName, R: Serializable { /* ... */ }

impl<T, R> serde::Serialize for Ty<T, R> where Ty<T, R>: Serializable { /* ... */ }