Add Ed25519 to FROST and remove expand_xmd for elliptic_curve's

Doesn't fully utilize ec's hash2curve module as k256 Scalar doesn't have 
FromOkm for some reason. The previously present bigint reduction is 
preserved.

Updates ff/group to 0.12.

Premised on https://github.com/cfrg/draft-irtf-cfrg-frost/pull/205 being 
merged, as while this Ed25519 is vector compliant, it's technically not 
spec compliant due to that conflict.

crypto/dalek-ff-group/src/lib.rs

@@ -22,8 +22,7 @@ use dalek::{
   }
 };
 
-use ff::{Field, PrimeField};
-use group::Group;
+use group::{ff::{Field, PrimeField}, Group};
 
 #[derive(Clone, Copy, PartialEq, Eq, Debug, Default)]
 pub struct Scalar(pub DScalar);