Faster Ed25519 implementation.
The ref10
Ed25519 code currently being used is rather slow, and does not support batch verification. Andrew Moon's Ed25519-donna
(https://github.com/floodyberry/ed25519-donna) is both faster and provides batch verification, in addition to the optimized Curve25519 scalar basepoint multiply that #9663 (moved) depends on.
We should use Ed25519-donna instead of ref10 for our Ed25519 implementation, at first with cross-checking/fallback, and eventually as our sole Ed25519 provider.