Cancelable and IdFree typeclasses.

Cancelable elements are a new way of proving local updates, by
removing some cancellable element of the global state, provided that
we own it and we are willing to lose this ownership.

Identity-free elements are an auxiliary that is necessary to prove that
[Some x] is cancelable.

For technical reasons, these two notions are not defined exactly like
what one might expect, but also take into account validity. Otherwise,
an exclusive element would not be cancelable or idfree, which is
rather confusing.