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.
This is used to skip downloading fonts in formats that we don't support.
Currently we only support TTF as far as I am aware.
The parts of a `src` are in a fixed order, unusually, which makes the
parsing more nesty instead of loopy.
