In some harmless places, checks for the return value of ECDSA and RSA
key generation and verification was omitted. Add them to keep the
compiler happy and to warn end users in case something is wrong.
This uses the portable Ed25519 library made by Orson Peters, which in turn uses
the reference implementation made by Daniel J. Bernstein.
This implementation also allows Ed25519 keys to be used for key exchange, so
there is no need to add a separate implementation of Curve25519.
