Add (partial) implementations for `char8_t`

[Flamefire]

This allows usages like

std::string u8ToGbk(const std::u8string &U8Str) {
    return boost::locale::conv::from_utf(U8Str, "GBK");    
}

as found in #190 See also #79

[codecov[bot]]

Codecov Report

Merging #192 (e6b02a4) into develop (0552ffc) will decrease coverage by 0.10%. The diff coverage is 90.55%.

Additional details and impacted files

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