Change ArrayBase.ptr to NonNull type

[jturner314]

Rust 1.25 introduces a NonNull type, which represents a pointer that must always be non-null. It would be nice to switch the ArrayBase.ptr field to type NonNull<S::Elem> to indicate that the pointer must always be non-null.

However, I'm not quite sure about the implications of NonNull<T> being "covariant over T". After reading this page in the nomicon, I think this is okay for Array, ArrayView, ArrayViewMut, and ArcArray, but I'm not sure that it's okay in general for arbitrary S: Data.

[bluss]

Sure, using NonNull should be good for us, don't see a problem with that. We need to remember to highlight the non-null requirement in ArrayView::from_shape_ptr. I'll have a think about if we already de facto require that pointer to be non-null (it certainly is non-null in all the safe ways of making an ArrayView).

[bluss]

NonNull was already listed over in #414, but I updated it to link to this issue as the specific one for that point.

[bluss]

(There is no “arbitrary S: Data” yet, because the Data trait is documented to be for our own purpose and we control all its implementations.)

[jturner314]

There is no “arbitrary S: Data” yet

Specifically, I'm thinking about the raw pointer array views we discussed in #388, for which NonNull might not make sense. There may also be other Data types that we want to add in the future.