Simplify Monero key image handling

crypto/frost/src/algorithm.rs

@@ -18,6 +18,7 @@ pub trait Algorithm<C: Curve>: Clone {
 
   /// Generate an addendum to FROST"s preprocessing stage
   fn preprocess_addendum<R: RngCore + CryptoRng>(
+    &mut self,
     rng: &mut R,
     params: &MultisigView<C>,
     nonces: &[C::F; 2],
@@ -119,6 +120,7 @@ impl<C: Curve, H: Hram<C>> Algorithm<C> for Schnorr<C, H> {
   }
 
   fn preprocess_addendum<R: RngCore + CryptoRng>(
+    &mut self,
     _: &mut R,
     _: &MultisigView<C>,
     _: &[C::F; 2],

crypto/frost/src/sign.rs

@@ -104,7 +104,7 @@ fn preprocess<R: RngCore + CryptoRng, C: Curve, A: Algorithm<C>>(
   serialized.extend(&C::G_to_bytes(&commitments[1]));
 
   serialized.extend(
-    &A::preprocess_addendum(
+    &params.algorithm.preprocess_addendum(
       rng,
       &params.view,
       &nonces