[FEA]: `cuda::span<T, Properties...>` a safe view over - heterogeneous - memory

[miscco]

We want to extent the property based design for safe heterogeneous memory accesses to view types.

This will most likely require us to implement an iterator type that does the right and then guard all direct memory accesses.

[bernhardmgruber]

I was thinking about span and using it over raw pointers in some of the CUB agents the other night. I wondered whether we need a customizable index type, so we can use a 32-bit integer to store the length. mdspan has this customization and IIRC the proposal authors were motivated by GPU codes as use case for this feature.