[RFC] Implement mdspan.

[Luc Grosheintz]

This patch completes the C++23 part of <mdspan> and must be applied
after the default_accessor patch.

A question for review: Should we add additional tests, that check
that the implementation works with custom layouts, accessors and
data_handle_types?

It's shared now to allow developers working on linear algebra to
proceed. I'll likely keep working on the tests. Therefore, until
default_accessor has been merged a detailed review might be premature.

Luc Grosheintz (1):
  libstdc++: Implement mdspan and tests.

 libstdc++-v3/include/std/mdspan               | 253 +++++++++++++
 libstdc++-v3/src/c++23/std.cc.in              |   3 +-
 .../23_containers/mdspan/class_mandate_neg.cc |  32 ++
 .../testsuite/23_containers/mdspan/mdspan.cc  | 348 ++++++++++++++++++
 4 files changed, 635 insertions(+), 1 deletion(-)
 create mode 100644 libstdc++-v3/testsuite/23_containers/mdspan/class_mandate_neg.cc
 create mode 100644 libstdc++-v3/testsuite/23_containers/mdspan/mdspan.cc

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2.49.0