Small fix in arithmetic_extension_special_cases (#140)

This is detecting the case where we multiply something by 1 and add 0.  In that case we can just return the thing being multiplied by 1. We were using the wrong constant to detect this.

Reduces the cost of `compute_evaluation` from 8 to 6 gates.

src/gadgets/arithmetic_extension.rs

@@ -143,7 +143,7 @@ impl<F: Extendable<D>, const D: usize> CircuitBuilder<F, D> {
                 }
             }
             if let Some(x) = mul_1_const {
-                if (x * const_1.into()).is_one() {
+                if (x * const_0.into()).is_one() {
                     return Some(multiplicand_0);
                 }
             }