There are still some holes, e.g. we leave surrogate pairs be
even though they are not a part of valid UTF-8, but this might
be for the better -- WTF-8 does support surrogate pairs inside
text.
Closes#1207
This allows reducing the amount of friends needed for its interface
and some extra tricks later.
The bad part is that the pointer can become invalidated via
calls to other StringRef's public methods, but c'est la vie.
Martin Hořeňovský