[makeset]

Note that GCC/libstdc++ (as of v15.2) does not yet implement std::mdspan [1], so it needs to be imported from another reference implementation like Kokkos [2].

[1] Merged in for v16: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107761

[2] https://github.com/kokkos/mdspan