Update mdspan to match upstream #800
Description
- mdspan was recently updated in Numerous fixes and updates kokkos/mdspan#19. Merge these changes into Draco's copy and test.
- This should fix build issues with intel/17.
Update 2020-9-07
- I've updated the ticket at Still cannot build mdspan under MSVC 2019 kokkos/mdspan#26.
- I've created a stand alone reproducer mini-repository to track this issue: https://github.com/kinetictheory/mdspan-msvc2019