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/tests/literal/mod.rs

@@ -1,2 +1,3 @@
-mod expand_message;
 mod kp256;
+#[cfg(feature = "ed25519")]
+mod ed25519;