This type unifies with any instance of classes that have a constructor which
* is public and
* unifies with the type used for type parameter `T`.
If a type parameter A is assigned to a type parameter B which is constrained
to Constructible<T>, A must be explicitly constrained to
Constructible<T> as well.
It is intended to be used as a type parameter constraint. If used as a real
type, the underlying type will be Dynamic.