Implement type passthrough

[Gankra]

As noted in #12 there's a need to express "use this native type of the target language". Another interesting case outside of Option is Rust's PhantomData. You want to confirm that

#[repr(transparent)]
struct Units<T>(u32, PhantomData<T>);

Is a valid pun for u32 even though it has an extra field, and it seems goofy as hell to have PhantomData builtin!