Simplify double-and-add code

Author: daxpedda

ed448-goldilocks/src/curve/scalar_mul/variable_base.rs

@@ -53,10 +53,7 @@ mod test {
             // XXX: Would be great if subtle had a From<u32> for Choice. But maybe that is not it's purpose?
             for bit in s_bits.into_iter().rev() {
                 result = result.double();
-
-                let mut p = ExtendedPoint::IDENTITY;
-                p.conditional_assign(point, Choice::from(bit as u8));
-                result = result.add(&p);
+                result.conditional_assign(&result.add(point), Choice::from(u8::from(bit)));
             }
 
             result