[roll pigweed] pw_span: Alias std::span when it is available or polyfilled

- If std::span is available and fully implemented, make pw::span an
  alias of it.
- If std::span is polyfilled, make pw::span an alias of it. It is not
  possible to make std::span an alias of pw::span since C++ library
  headers contain forward declarations of std::span, even when building
  with C++17.
- Otherwise, define pw::span as a separate class to avoid adding
  anything to the std namespace.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/100201

https://pigweed.googlesource.com/pigweed/pigweed
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Change-Id: Ib0fb9b8b4e76a8386df2561b2ede90848ea93409
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/zephyr-integration/+/100189
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1 file changed
tree: c2c17574a3908858c46a3304d23c4bdb0abb51a8
  1. .gitmodules
  2. test.sh