Make OwnedRepr an "aliasable owner" instead of using Vec

[bluss]

The owned array has used a Vec<T> that it owns, and kept a separate "head pointer" ptr on the side; the head pointer points to the first element in the array, somewhere inside the vector's data.

The Vec uses a "unique" owning pointer - not a building block that is available outside std - so just like Box, it should in principle not be possible to modify the vector's data through anything else than its own data pointer. Our head pointer can violate this.

We replace the Vec with a deconstructed vector; it's equivalent but uses NonNull<T> for its pointer, so it's an aliasable owner.

This does not change how owned arrays are allocated, only how it is modelled in the type system. The change affects all the owned arrays (Array, ArcArray, CowArray).

cc #796

In principle, this problem is described in issue Kimundi/owning-ref-rs#49

This soundness concern does not currently seem to yield any problems in practice, because the compiler does not take advantage of the properties of Vec's unique pointer.