case conversion algorithm 大小写转换算法
void to_lower(mutablecollection&, std::local&=std::local());
void to_upper(mutablecollection&, std::local&=std::local());
sequence to_lower_copy(const sequence&, std::local&=std::local());
sequence to_upper_copy(const sequence&, std::local&=std::local());
outputiterator to_lower_copy(outputiterator, const collection&, std::local&=std::local());
outputiterator to_lower_copy(outputiterator, const collection&, std::local&=std::local());