The internal Baseiter type underlies most of the ndarray iterators, and it used *mut A for element type A. Update it to use NonNull<A> which behaves identically except it's guaranteed to be non-null and is covariant w.r.t the parameter A.
Add compile test from the issue.
The second commit continues the "conversion", using NonNull more consistently
in Baseiter's Iterator impl.
The internal Baseiter type underlies most of the ndarray iterators, and it used
*mut Afor element type A. Update it to useNonNull<A>which behaves identically except it's guaranteed to be non-null and is covariant w.r.t the parameter A.Add compile test from the issue. The second commit continues the "conversion", using NonNull more consistently in Baseiter's Iterator impl.
Fixes #1290