[kroplabeskidu]

Description

Item 18 from #6159. array_t::at() and mutable_at() divide the byte offset by itemsize() and then index a T *. That only works if the stride is a multiple of sizeof(T). With a field of a packed structured array, it reads or writes the wrong bytes:

packed = np.zeros(3, dtype=[("a", np.uint16), ("b", np.uint8)])
packed["a"] = [10, 20, 30]
m.at_t(packed["a"], 1)  # was 5120, now 20

Both now use data(index...) and mutable_data(index...), which apply the byte offset directly and have the same bounds, negative-index and writeable checks.

index_at() is unchanged. It returns an item count, which still can't represent such strides.

Suggested changelog entry:

• array_t::at() and mutable_at() now return the right element when a stride is not a multiple of the item size.