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 :^)
This commit doesn't include support for FETCH BODY, because it's a bit
big already. Rest assured, FETCH is the most complicated IMAP command,
and we'll go back to simple boring ones shortly.
