Sync this lib with crypto/ed25519

[ValarDragon]

We need to sync this repo back with crypto/ed25519. (The commits on crypto/ed25519 are from the same author) They've added updates that we need to add here. (Support for RFC 8032 - prevents signature malleability). I'll try to write a PR for this soon.

[liamsi]

Perfect, thanks!
see also: #2 (comment)