mdspan

Uses index_type and size_type consistently across mdspan and extents.

Renames mdspan::pointer to data_handle and contiguous to exhaustive.

Adds the missing empty() to mdspan.

Fixes layout_stride::mapping's default constructor for static extents.

Adds std::submdspan for slicing an mdspan.

Adds layout_left_padded and layout_right_padded for BLAS-style padded layouts.

Improves class template argument deduction for mdspan.

Adds dims<Rank> so dextents need not spell the index type.

Adds std::aligned_accessor, expressing pointer over-alignment for mdspan.

Fixes the submdspan specification for C++26.

Adds bounds-checked mdspan::at().

Future-proofs the submdspan_mapping customization point.

Splits strided_slice into extent_slice and range_slice.

Adds std::copy and std::fill overloads for mdspan.