feat: impl (de)ser for PhantomData

[austinabell]

closes #30

There is another option, which is to serialize a unit struct for the phantom data, similar to serde does by default. If this is done, possibly also schema can be added for it, otherwise, I don't think schema would work for this.

[ailisp]

Looks good! Is there a DoS attack factor for struct A { pd: PhantomData }; deserialize::<Vec<A>>?

[austinabell]

Looks good! Is there a DoS attack factor for struct A { pd: PhantomData }; deserialize::<Vec<A>>?

Yeah could be, but only in the case where a deserialized object contains nothing but PhantomData, which should be tackled as a larger issue, relating to the zero size workaround that exists. I think it might be hard to rationalize creating another workaround to limit the symptoms of the problem like the other, though