A general implementation of the sum type. Meant to be a generic counterpart to tuples.
Cheat Sheet
use *;
/// Often, the type of iterator is conditional to some input value. Typically,
/// to unify the return type, one would need to implement a custom iterator, but
/// here `orn` types are used instead.
/// Using the `tn` methods, items of an `Or` value can be retrieved.
/// Using the `At<I>` trait, items of an `Or` value can be retrieved
/// generically.