Avoid unwanted sign extensions from MSVC in is_utf8.

[mwinterb]

Microsoft's constexpr evaluator treats the type of micro[0] and micro[1] as
plain char, and so sign extends before comparing them to ints.
The normal compiler, including the optimizer, does not fail in this way,
so this is merely a "future proof" change in case someone uses is_utf8()
in a constant expression.

[mwinterb]

The smallest repro pointing out the issue is this: https://gcc.godbolt.org/z/xTxPfzqz9. /J is how Microsoft spells -funsigned-char. All compiles should return 1 from main() to indicate agreement between constant evaluation and "runtime" evaluation, but mostly default options do not do that.
I couldn't find any decent places to forcibly test this as none of the tests specify /utf-8, and it seems ODR-violating to just compile the tests with that and rude to add the option unconditionally when building the static library.
Another fix is just to make is_utf8 non-constexpr, but keeping the comment that it's explicitly non-constexpr to avoid Microsoft's bug.

[vitaut]

Thanks for the PR. Looks good, just two nits.

[vitaut]

nit: Please make it a proper sentence:

  // Avoid buggy sign extensions in MSVC's constant evaluation mode
  // https://developercommunity.visualstudio.com/t/C-difference-in-behavior-for-unsigned/1233612.

[vitaut]

Thanks!