Address private key leak vulnerabilty in ed25519-dalek

[arjanz]

Article at https://github.com/MystenLabs/ed25519-unsafe-libs mentions potentially unsafe ed25519 signature libraries that allow a public api where secret and public key can be provided independently as signing function inputs. Misuse of these public apis can result to private key exposure.

ed25519-dalek (RUST crate used for the bindings) is on that list and currently a PR is created addressing this issue: dalek-cryptography/ed25519-dalek#205

Will upgrade bindings as soon this is merged

[arjanz]

Followed Substrate by moving to ed2219-zebra, created new bindings: https://github.com/polkascan/py-ed25519-zebra-bindings