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.

crypto/frost/src/tests/mod.rs

@@ -82,7 +82,7 @@ pub fn key_gen<R: RngCore + CryptoRng, C: Curve>(
       }
       our_secret_shares.insert(*l, shares[&i].clone());
     }
-    let these_keys = machine.complete(our_secret_shares).unwrap();
+    let these_keys = machine.complete(rng, our_secret_shares).unwrap();
 
     // Verify the verification_shares are agreed upon
     if verification_shares.is_none() {