Reconcile `utf8_support` handling in std backend

[Flamefire]

Document the meaning of the utf8_support enum and refactor/change usages to be consistent and logical.
Add comments where required.

• Remove the unused utf8_support::native which is only checked for but never set
• Rename native_with_wide to native after that is now free for use. This makes the meaning clear as there is no native_without_wide.
• Move the Windows-codecvt workaround directly where it matters/is used to not abuse utf8_support for this which was confusing

Closes #171

@artyom-beilis Please do a quick check that I didn't miss anything or misunderstood something you wrote in #171

[artyom-beilis]

I added few comments. :-)

[codecov[bot]]

Codecov Report

Merging #173 (ac05f1d) into develop (aa58a42) will increase coverage by 0.37%. The diff coverage is 100.00%.

Additional details and impacted files

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