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/frost/src/lib.rs

@@ -13,7 +13,7 @@ mod schnorr;
 pub mod key_gen;
 pub mod algorithm;
 pub mod sign;
-#[cfg(any(test, feature = "p256", feature = "k256"))]
+#[cfg(any(test, feature = "curves"))]
 pub mod curves;
 
 pub mod tests;