Use cryptonite Ed25519 primitives

[jml]

We have our own implementation of Ed25519. We really should avoid this, because less code is better, and something in cryptonite is likely to be better maintained.

haskell-crypto/cryptonite#202 added the primitives, and it's been released in 0.25.

We should give this a try. Need to make sure it stays compatible with python-spake2.

This would be a good beginner ticket.