Implements iterators for the Span class to be compatible with std::span

[marcemmers]

Summary of changes

Implements iterators for the Span class to be compatible with std::span.
This makes it possible, among other things, to use range based for loops on a span.

Impact of changes

Migration actions required

Documentation

It might be useful to add this to the API reference but its not necessary.

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Pull request type

[] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[X] 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-core @ARMmbed/mbed-os-maintainers please review.

[pan-]

Thanks @marcemmers that is very useful for use in algorithms and range-based for loop. One minor thing, could you include the <iterator> header ?

Pull request has been modified.

[0xc0170]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 6 of 6 test jobs passed
Build number : 1
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