Adventures in <mdspan>#

These are some “stories” from working on <mdspan> for libstdc++:

• Simplifying branch conditions in extents::extent.
• Static for-loop in extents::operator==.

Many thanks to Tomasz Kaminski (tkaminsk@redhat.com) for his useful insights during the review process for libstdc++ and for teaching me the modern way of doing compile-time computations without the complications of recursive structs.