Correct clippy warnings

Currently intended to be done with:
cargo clippy --features "recommended merlin batch serialize experimental 
ed25519 ristretto p256 secp256k1 multisig" -- -A clippy::type_complexity 
-A dead_code

crypto/multiexp/src/batch.rs

@@ -24,7 +24,7 @@ where
     pairs: I,
   ) {
     // Define a unique scalar factor for this set of variables so individual items can't overlap
-    let u = if self.0.len() == 0 {
+    let u = if self.0.is_empty() {
       G::Scalar::one()
     } else {
       let mut weight;