All issues are now resolved.
This reverts commits
3802e8ecb9 (CI: Test only building the library on Windows)
36f718e12d (CI: Test only building the library when using ICU)
Windows uses a 2-Byte `wchar_t` which effectively is UCS-2,
i.e. only codepoints up to U+FFFF.
So decoding an UTF-8 file with codepoints higher than that will fail.
As most Windows APIs now accept UTF-16 encoded `wchar_t` it makes sense
to also use that for the `codecvt` generated by the std-backend.
This is actually disallowed to be used for `std::basic_filebuf` but it
works in practice, so use it anyway.
Closes#71
Gets rid of the `test_` prefix in the filenames and avoids confusing those with the actual tests especially by renaming the test framework file from `test_locale.hpp` to `unit_test.hpp`
