experiment with UTF-8/16 C++ iterators

[markusicu]

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• Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-_____
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Notice: the branch changed across the force-push!

• icu4c/source/common/unicode/utf16cppiter.h is different
• icu4c/source/common/unicode/uversion.h is no longer changed in the branch
• icu4c/source/test/intltest/intltest.vcxproj is different
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• icu4c/source/test/intltest/itutil.cpp is different
• icu4c/source/test/intltest/Makefile.in is different
• icu4c/source/test/intltest/utfcppitertest.cpp is different

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[markusicu]

Hi @eggrobin I think this is worth taking another look. I rebased on recent main, made changes from our discussions, and I think this looks roughly like a reasonable validating, forward-only (so far) Unicode 16-bit-string code point iterator. It no longer tries to be clever: It no longer reads & validates the code point while iterating, and no longer stores the result in the iterator.

Plenty of TODOs and questions left, but I would appreciate feedback on the shape of what I've got so far.