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 :^)
Rather than casting the FixedPoint to double, format the FixedPoint
directly. This avoids using floating point instruction, which in
turn enables this to be used even in the kernel.
