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.
There are a whole bunch of SVG attributes, and we shouldn't mix them in
with the HTML attributes. This patch adds some of them to the new
namespace, but there are more to be added. :^)