I would love to have a lawyer chime in on this. It seems like the license at the root should be enough to say that you are applying the license to all the code.
Actually, there's been another advance since Ryu: Schubfach by Raffaello Giulietti and its optimized derivative Dragonbox by Junekey Jeon.
Here's a cool page with implementations and a benchmark: https://github.com/abolz/Drachennest/