[public] Provide methods on PrivateKey and PublicKey for signing/verification
[public] Provide methods on PrivateKey and PublicKey for signing/verification

- Add convenience method PrivateKey::sign, unlocking the pattern
  key.sign(message)
- Add convenience method PublicKey::verify, unlocking the pattern
  key.verify(message, signature)

Change-Id: Iaf69741bd72924f0db355d665da778d69c31fae8

google/mundane,google/mundane,google/mundane