Added support for CuArray via Adapt.jl
This second attempt, uses a very light-weight implementation to add CuArray support to ShiftedArrays.jl.
It is based on using an Extension Package, such that ShiftedArrays does not drag in any extra packages, but if CUDA.jl is present, the adaptation is used.
Note that the show() function was specified by CuArrays preventing errors via the @allowscalar macro.
The tests were extended to CuArray usage, but this has to be enabled manually in the runtests.jl file.
Thanks for the PR, I've left a review with some suggestions. I'll be traveling for the holidays so I might be slow in responding for the next couple of weeks.
... there were a couple of typos. Now the show adaptation is commented out and the typos were fixed. The tests run fine. Seems we are getting closer.