Implement a binary search for BatchVerifier blame

Adds helper functions to verify and, on failure, blame, which move an 
unwrap from callers into multiexp where it's guaranteed to be safe and 
easily verified to be proper.

Closes https://github.com/serai-dex/serai/issues/10.

crypto/frost/src/key_gen.rs

@@ -225,10 +225,7 @@ fn complete_r2<R: RngCore + CryptoRng, C: Curve>(
 
     batch.queue(rng, *l, values);
   }
-
-  if !batch.verify() {
-    Err(FrostError::InvalidCommitment(batch.blame_vartime().unwrap()))?;
-  }
+  batch.verify_with_vartime_blame().map_err(|l| FrostError::InvalidCommitment(l))?;
 
   // TODO: Clear the original share