The reference type, `RefBase<A, D, R>`, is parameterized by its element
type, a dimensionality type, and its "referent" type. This last type
is only a `PhantomData` marker, and acts to differentiate references
that are "safe" from those that are "raw", i.e., references that come
from an array whose storage is `Data` vs an array whose stoarge is
`RawData`. The `DerefMut` implementation contains a check of uniqueness,
guaranteeing that an `&mut RefBase` is safe to mutate.

This comes with one breaking change and one important internal library
note. The breaking change is that `ArrayBase` no longer implements `Copy`
for views. This is because a `RefBase` does not know where it is pointing,
and therefore cannot implement `Copy`; this, in turn, prohibits
`ArrayBase` from implementing `Copy`. Users will now have to explicitly
invoke `Clone` in the same way that they do for non-view arrays.

The important library note has to do with the `.try_ensure_unique` check
that occurs in the `DerefMut` implementation. The library's methods are
allowed to *temporarily* break the invariant that an array's `ptr`
is contained within `data`. The issue comes when trying to then alter
`ptr`, `shape`, or `dim` while that invariant is broken. Code was
using `self.ptr = ...` (where `self` is `ArrayBase`), which triggers
`DerefMut`, which calls `try_ensure_unique`, which panics on the
pointer inbounds check. As a result, code that is altering `RefBase`
fields while the array's invariants are broken must be sure to write
`self.aref.ptr = ...` instead.

… Copy

…to RefBase and LayoutRef

…ow to write functionality with the new types

… bullets on the new types

I think I accidentally moved this over to ArrayRef, but we're not sure it should still require DataOwned

This is unfortunately out of our control: users can select their blas implementations, and those implementations may not be MSRV 1.64, like ndarray itself. So we should add some documentation that makes this clear (or bump our MSRV). For now, we'll just add a gate in our test driver to handle this case