Enable no_std on transcript

Removes the Vec challenge for an associated type.

Fixes the merlin feature which was horribly broken.

Also adds no_std to dalek-ff-group.

crypto/frost/src/algorithm.rs

@@ -62,6 +62,8 @@ pub trait Algorithm<C: Curve>: Clone {
 #[derive(Clone, Debug)]
 pub struct IetfTranscript(Vec<u8>);
 impl Transcript for IetfTranscript {
+  type Challenge = Vec<u8>;
+
   fn domain_separate(&mut self, _: &[u8]) {}
 
   fn append_message(&mut self, _: &'static [u8], message: &[u8]) {

crypto/frost/src/sign.rs

@@ -164,7 +164,7 @@ fn sign_with_share<C: Curve, A: Algorithm<C>>(
     transcript.append_message(b"message", &C::hash_msg(&msg));
 
     // Calculate the binding factor
-    C::hash_binding_factor(&transcript.challenge(b"binding"))
+    C::hash_binding_factor(transcript.challenge(b"binding").as_ref())
   };
 
   // Process the addendums