Address `wchar_t` about cpp-string-template

[acd1034]

• テンプレート引数に応じて文字リテラルの型を切り替える
• Microsoft STL

template <typename CharT>
struct choose_literal; // not defined

template <>
struct choose_literal<char> {
    static constexpr const char* choose(const char* s, const wchar_t*) {
        return s;
    }
};

template <>
struct choose_literal<wchar_t> {
    static constexpr const wchar_t* choose(const char*, const wchar_t* s) {
        return s;
    }
};

#define TYPED_LITERAL(CharT, Literal) (choose_literal<CharT>::choose(Literal, L##Literal))