simplify String?, ContiguousString? and SizedString? protocols

String? is an alias to Sequence? of Char, and nothing more.
UniString? protocol

UniString? is a string of UniChar

`UTF8[String]` and `Vector[UniChar]` are examples of `UniString?`
overload printTo for UniString?

Print `UniString?` as UTF-8-encoded string instead of comma-separated
characters