Investigate adding structured dtype support to CuPy

NumPy supports limited forms of custom types via [structured dtypes ](https://numpy.org/devdocs/user/basics.rec.html).

But CuPy does not. One of the benefits of `cuda.parallel` is that it could enable this.

We should investigate what it would take to make this work.

This issue can be closed after we have created another tracking issue breaking down the concrete work breakdown of what all would go into making structured dtypes work in cupy array algorithms.