ed25519: sign+verify support

[phlip9]

By any chance is ed25519 sign/verify support on the roadmap? I see x25519 key exchange is in-scope, so hoping this isn't too much of a stretch goal.

Would otherwise love to use this library to fix our ring build headaches, but ed25519 support is the major missing piece for us. Thanks!

[ctz]

Yes that is definitely on my mind. Hasn't been a priority thus far, due to the low level of use in the web. But s2n-bignum has everything required for this.