Avoid unwanted sign extensions from MSVC in is_utf8.
Avoid unwanted sign extensions from MSVC in is_utf8.

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.

alabuzhev/fmt,alabuzhev/fmt,cppformat/cppformat,alabuzhev/fmt,cppformat/cppformat,cppformat/cppformat