Instead of calling to_lowercase() on two strings for every step while
iterating over the HTMLCollection returned by getElementsByTagName(),
we now cache the lowercased tag name beforehand and reuse it.
2.4x speed-up on WebKit/PerformanceTests/DOM/DOMDivWalk.html
DeprecatedFlyString relies heavily on DeprecatedString's StringImpl, so
let's rename it to A) match the name of DeprecatedString, B) write a new
FlyString class that is tied to String.
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 :^)
