ed25519-dalek known bugs

Versions of `ed25519-dalek` prior to v2.0 model private and public keys as separate types which can be assembled into a `Keypair`, and also provide APIs for serializing and deserializing 64-byte private/public keypairs. Such APIs and serializations are inherently unsafe as the public key is one of the inputs used in the deterministic computation of the `S` part of the signature, but not in the `R` value. An adversary could somehow use the signing function as an oracle that allows arbitrary public keys as input can obtain two signatures for the same message sharing the same `R` and only differ on the `S` part. Unfortunately, when this happens, one can easily extract the private key. Revised public APIs in v2.0 of `ed25519-dalek` do NOT allow a decoupled private/public keypair as signing input, except as part of specially labeled "hazmat" APIs which are clearly labeled as being dangerous if misused.