We have a new, improved string type coming up in AK (OOM aware, no null
state), and while it's going to use UTF-8, the name UTF8String is a
mouthful - so let's free up the String name by renaming the existing
class.
Making the old one have an annoying name will hopefully also help with
quick adoption :^)
Previously this was handled implicitly, as our implementation of Tar
would just stop processing input as soon as it found something invalid.
However, since we now error out as soon as something is found to be
wrong, we require proper handling for zero blocks, which aren't actually
fatal.
