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 :^)
The Buffer files had contained both the ResampleHelper and the
sample format utilities. Because the Buffer class (and its file) is
going to be deleted soon, this commit separates those two things into
their own files.