This is an early sketch of proposal for std::code_unit_sequence which is
intended to directly replace std::u8string, std::u16string and
std::u32string.

It is based on std::vector so I removed most of useless member functions
that std::basic_string has. The API is much cleaner.

It uses strong types that I mentioned earlier. Those are classes and
since they don't exist in C, there is no NUL-terminator. Finally, owning
string and view are unified.

As I plan to write many more proposals, I'm gonna create a single
repository for them soon.