Support curve25519

[Erik1000]

This issue is blocked, because ed25519-dalek does not allow us to reconstruct a key from its individual points (see the RFC). We'd need to use Keypair::from_bytes and bring the bytes in the correct format which is kind of tricky.

[Erik1000]

We are able to use Keypair::from_bytes but ed25519-dalek has outdated dependencies and other ed25519 implementations don't allow us to sign prehashed messages which is needed by our Signer trait. The outdated dependencies of ed25519-dalek (digest to be precise) break the trait bounds of our Signer.

[Erik1000]

Now, we can use (SigningKey::sign_prehashed)[https://docs.rs/ed25519-dalek/2.0.0-rc.3/ed25519_dalek/struct.SigningKey.html#method.sign_prehashed] but since this is ed25519*ph* the output is different from ed25519. This however is only because of the context used for domain separation but I think we cannot bypass this.