Provide overloads with wchar_T in non Windows systems

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For instance this code doesn't compile on Mac:

// string get(const WCHAR* defv) const
get(L""); // Cannot convert wchar_t const* to WCHAR

[c-smile]

You can use either:

get(u"")

Or there is WSTR(string literal) for that:

get(WSTR(""))

See: https://en.cppreference.com/w/cpp/language/string_literal

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u"" has type const char16_t[N] while there is no such overload. char16_t is a distinct type.