Support use in CUDA device-side code #17

nVIDIA's CUDA is a popular ecosystem for general-purpose GPU programming. Essentially, in CUDA, you write kernels to be executed on a GPU using a slightly-restricted variant of C++. However, you need to annotate functions which run the the GPU-device-side as __device__. (For constexpr function this can be skipped, but only with a certain compiler flag which shouldn't be relied upon.) Also, any host-side code of the standard C++ library is not usable.

I would like to ask that this span implementation be adapted for use with CUDA. I've done something similar for std::array, although there's a bit of unnecessary boilerplate in my additions there.