Implement Ord on PublicKey

[Neotamandua]

Summary

Having Ord directly available on PublicKey makes some use cases easier, for example using PublicKeys as keys in BTreeMap and BTreeSet or being able to sort a list that contains PublicKeys.

Example: https://github.com/dusk-network/rusk/blob/46f637110696c9e7816a6746aa4a9b4453735a7d/contracts/transfer/src/state.rs#L600