[review] Klemens Condition 3

[mborland]

Make array_ref and subarray the same type

[correaa]

Although array_ref can be used in any place subarray is used, the opposite is not true.
array_ref provides more guarantees; that the data contained is contiguous, and that the block of memory has no gaps.

This way array_ref has .data_elements() but subarray doesn't.