prop224: Validate ed25519 pubkeys to remove torsion component

We should validate ed25519 pubkeys used in prop224 (like the blinded key) to make sure there is no torsion components. In the case of the onion address ed25519, this ensures that there are no equivalent onion addresses due to the torsion component.

Please see: https://lists.torproject.org/pipermail/tor-dev/2017-April/012164.html https://lists.torproject.org/pipermail/tor-dev/2017-April/012213.html