Contributions after C++23 mdspan.

[Luc Grosheintz]

> Firstly, thank you very much for your contributions, they are really
> appreciated.

Thank you! I'm glad to hear it.

> One option would be to continue with mdspan later mappings, but I will
> understand
> if you would like to take a break from the topic.

Locally, I have "complete" patches for: default_accessor and mdspan.
I'll start sending them soon. I don't want to send too much at once.

This is (a roundabout version of) the email asking if I can start
preparing layout_{left,right}pad (and then keep going until the
we finish all md-related C++26 features)?