Add Null back-end

[sampollard]

We also need to include the Null back-end. Currently it is a branch of Kokkos on @vlkale 's fork of Kokkos - regardless of where this ends up, we should make it accessible here. Perhaps submodules are the right way to do this.

[vlkale]

From looking at this more and through discussing with Kokkos team members, we may want to actually make xstack-klokkos either

1. a submodule of Kokkos-core
2. A submodule of Kokkos-tools
3. another subproject in the Kokkos Ecosystem(e.g., kokkos clang-tidy, kokkos-core, kokkos-tools)

Note that this github issue suggests we are trying to do the converse of option 2.

Option 2 is the most desirable and option 3 is probably the most straightforward.

Long-term, we would like to have Kokkos application programmers klee testing be part of CI tests of Kokkos applications. The option chosen should strive towards this vision.