Rename Utf8Buf::new into Utf8Buf::wrap

Better not give an impression that something new is allocated.
This is used when a previously allocated pointer is being wrapped.
Bikeshed Utf8* into UTF8*
Unsafe creation methods for UTF8Str
Bikeshed Error::init() into Error::unset()
More utility methods to UTF8Str, into_string faster
error::Error gets method message()