Use built-in sum instead of writing our own ad-hoc version (#738)

Extracted from #736 for ease of
review.

ceno_zkvm/src/uint/arithmetic.rs

@@ -274,9 +274,7 @@ impl<const M: usize, const C: usize, E: ExtensionField> UIntLimbs<M, C, E> {
                 .into_iter()
                 .unzip();
 
-        let sum_expr = is_equal_per_limb
-            .iter()
-            .fold(Expression::ZERO, |acc, flag| acc.clone() + flag.expr());
+        let sum_expr = is_equal_per_limb.iter().map(ToExpr::expr).sum();
 
         let sum_flag = WitIn::from_expr(|| "sum_flag", circuit_builder, sum_expr, false)?;
         let (is_equal, diff_inv) =

mpcs/src/sum_check/classic/coeff.rs

@@ -49,9 +49,7 @@ impl<E: ExtensionField> ClassicSumCheckRoundMessage<E> for Coefficients<E> {
     }
 
     fn sum(&self) -> E {
-        self[1..]
-            .iter()
-            .fold(self[0].double(), |acc, coeff| acc + coeff)
+        self[0] + self[..].iter().sum::<E>()
     }
 
     fn evaluate(&self, _: &Self::Auxiliary, challenge: &E) -> E {