Make ed25519 and ed448 usable for Golang interfaces

Hi!

Thanks so much for all the work on this!

So, the implementation for ed25519 and ed448 expose a certain API that is quite similar to the way golang handles it in its libraries (ecdsa, rsa, and ed25519). However, when I tried to use it for the crypto signer interface, it panics due to:

```
panic: interface conversion: ed25519.PrivateKey is not crypto.Signer: missing method Public [recovered]
panic: interface conversion: ed25519.PrivateKey is not crypto.Signer: missing method Public
```

I was wondering if we could create a method `func (priv *PrivateKey) Public() crypto.PublicKey` (in similar fashion as https://golang.org/pkg/crypto/ecdsa/#PrivateKey.Public and https://golang.org/pkg/crypto/ed25519/#PrivateKey.Public) so the APIs can be used for other interfaces as the crypto signer...