Add ed448

[baloo]

This is a rebase of #1121

I ended up moving some of the implementation to primeorder

[tarcieri]

Like I said in #1121, it's weird to use primeorder with ed448 because ed(wards)448 is not a prime order curve. To the extent there's reusable functionality that doesn't depend on it, we should maybe try to find a better home for such functionality, like elliptic-curve

It'd be good to keep ed448 close to the original crate, and we can document some work items to do as followups.

[baloo]

Those were mostly the mul macros, but I haven't got time to refactor that last commit. I'll give it another shot.