Supporting 'mdspan'.

[Luc Grosheintz]

Dear libstdc++ devs,

I would like to assist in the process of integrating `mdspan` into
libstdc++. In order to prepare for this project and get familiar with
how libstdc++ is setup, I've started implementing `std::extents`; and
writing the code that tests the implementation.

One option I see, is that I continue refining the implementation until I
can't find anymore tests that could expose a defect; and then send in a
patch for `std::extents`; and if successful proceed to the layouts and
eventually `mdspan` itself.

However, I'd also like to ask the libstdc++ developers if they would
prefer that I work on something else that advances the implementation of
`mdspan`; or on something else all together.

Kind regards,
Luc