<mdspan>: extents improvements

[JMazurkiewicz]

• Guard _STL_VERIFY with #if _CONTAINER_DEBUG_LEVEL > 0 (towards <mdspan>: Completely rework std::extents #3586 (comment)). I believe that this is the best solution, because:
  • The checks are cheap,
  • The checks don't require ABI-changing bookkeeping.
• Improve extents' constructors:
  • extents(other extents) constructor avoids using for loop and uses index_sequence instead,
  • extents(exts...) constructor avoids creating temporary arrays and using for loop. Instead, it puts exts... in a tuple and then constructs _Dynamic_extents array directly,
  • extents(exts...) now always checks elements in exts when it is possible. Previously it checked element when all elements were "checkable" (were integral),
  • extents(span/array) is more efficient.
• Add extra internal checks in _(Fwd/Rev)_prod_of_extents,
• Improve test code:
  • Add parentheses around std::min in test_mdspan_support.hpp,
  • Add extra death test to verify new extents(exts...)'s behaviour.

[StephanTLavavej]

Looks great, thank you! ✨