mstd::span rename index_type to size_type according to spec

[marcemmers]

Summary of changes

Renames the size type in mstd::span from index_type to size_type according to the spec, see https://en.cppreference.com/w/cpp/container/span member types. Ran into this issue when trying to implement the c++23 spanstream classes that are using the size_type from span.

Impact of changes

This is breaking for everyone that used the index_type of the span. No functional changes though.

Migration actions required

Rename all usages of span<>::index_type to span<>::size_type

Documentation

None

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Pull request type

[X] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[] No Tests required for this change (E.g docs only update)
[X] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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Reviewers

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[ciarmcom]

@marcemmers, thank you for your changes.
@ARMmbed/mbed-os-maintainers please review.

[pan-]

@marcemmers Maybe the previous alias can kept to avoid breaking existing applications.

[marcemmers]

@marcemmers Maybe the previous alias can kept to avoid breaking existing applications.

That's possible of course. Would you prefer it if I made the index type deprecated then? Like:

using index_type [[deprecated("Use size_type instead.")]] = size_t;

[0xc0170]

@marcemmers let's do nonbreaking fix

[0xc0170]

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[mbed-ci]

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