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

@@ -1,3 +1,5 @@
+#![no_std]
+
 use core::{
   ops::{Deref, Add, AddAssign, Sub, SubAssign, Neg, Mul, MulAssign},
   borrow::Borrow,