Structured bindings for `extents`

The extents class could provide support for structured bindings to improve its usability. This way, we could transform your example from:

  stdex::mdspan m{d.data(), stdex::extents{3, 3}};
  for (std::size_t i = 0; i < m.extent(0); ++i)
    for (std::size_t j = 0; j < m.extent(1); ++j) // was m.extent(0)

to:

  stdex::mdspan m{d.data(), stdex::extents{3, 3}};
  const auto [rows, cols] = m.extents();
  for (std::size_t i = 0; i < rows; ++i)
    for (std::size_t j = 0; j < cols; ++j)

See your modified example: https://godbolt.org/z/enYcvbG8a

It's not a huge improvement, but with increasing rank, it can save some redundant mentioning of m.extents(...). Or prevent accidentially passing the wrong index ;)
It also helps when I pass an instance of extents around without its surrounding mdspan. Then instead of:

const auto x = e.extent(0);
const auto y = e.extent(1);
const auto z = e.extent(2);

I can write:

const auto [x, y, z] = e;

The issue names no implementation file or test. Start by reading the extents API and the linked Compiler Explorer example, then identify the existing test location for extents behavior; done means an extents instance supports the shown structured-binding forms while preserving its extent values.