<mdspan>: Product code review

[StephanTLavavej]

• Unconditionally strengthen mdspan::is_always_MEOW().
  • They return mapping_type::is_always_MEOW() which is required to be a constant expression of type bool (N4950 [mdspan.mdspan.overview]/3, [mdspan.layout.policy.reqmts]/1, [mdspan.layout.reqmts]/22,24,26). The Standard should simply be enhanced to say noexcept here.
  • Also introduce constexpr bool _Result to make this extra clear and improve debug codegen.
• Drop strengthened comment; _Access_impl() is _Ugly.
• Follow N4950 [mdspan.mdspan.cons]/8.1 by saying const _OtherIndexType& in this constraint.
• In default_accessor, define access() before offset() to follow the order of N4950 [mdspan.accessor.default.overview].
• layout_stride::template mapping is unnecessary; layout_stride is not a template.
• Add explicit to the deduction guide extents(_Integrals...), depicted by N4950 [mdspan.extents.overview] and [mdspan.extents.cons]/12.
• Style: Reorder the template parameters for _Contains_multidimensional_index() to match its function parameters; it's always called with template argument deduction.
• Strengthen operator[] for span/array. Fine-grained commits for clarity:
  • Step 1: Move _Multidimensional_access() up.
  • Step 2: Add conditional noexcept to _Multidimensional_access(), not commented as strengthened because it's _Ugly.
    • It's now identical to the real multidimensional subscript operator except for the name, the comment, and the intentional lack of constraints.
  • Step 3: Add as_const() to the array overload, so it exactly matches the span overload.
    • This is what N4950 [mdspan.mdspan.members]/6 depicts. It was being implicitly skipped before (const array& has the same effect), but adding this will make the following unification clearer.
  • Step 4: Replace lambdas with _Multidimensional_subscript().
    • This unifies them by wrapping the array in a span.
  • Step 5: Add conditional noexcept to _Multidimensional_subscript (not commented, it's _Ugly), then strengthen operator[] for span/array.
    • This brings the span/array subscript operators up to parity with the multidimensional subscript operator.