Add a batch verifier to multiexp, along with constant time variants

Saves ~8% during FROST key gen, even with dropping a vartime for a 
constant time (as needed to be secure), as the new batch verifier is 
used where batch verification previously wasn't. The new multiexp API 
itself also offered a very slight performance boost, which may solely be 
a measurement error.

Handles most of https://github.com/serai-dex/serai/issues/10. The blame 
function isn't binary searched nor randomly sorted yet.

coins/monero/tests/frost.rs

@@ -62,7 +62,7 @@ pub fn generate_keys() -> (HashMap<u16, MultisigKeys<Ed25519>>, Scalar) {
       }
       our_secret_shares.insert(*l, shares[&i].clone());
     }
-    keys.insert(*i, machine.complete(our_secret_shares).unwrap().clone());
+    keys.insert(*i, machine.complete(&mut OsRng, our_secret_shares).unwrap().clone());
   }
 
   let mut group_private = Scalar::zero();