Simplify some code

Author: daxpedda

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

@@ -9,10 +9,7 @@ pub(crate) fn double_and_add(point: &ExtendedPoint, s_bits: [bool; 448]) -> Exte
     // 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

ed448-goldilocks/src/field/element.rs

@@ -476,7 +476,7 @@ impl FieldElement {
         let e = b * c;
 
         let mut a = n * e;
-        a.conditional_negate(!Choice::from(a.0.retrieve().bit(0)) ^ square);
+        a.conditional_negate(!a.is_negative() ^ square);
 
         let c = e * ONE_MINUS_TWO_D;
         let b = c.square();

ed448-goldilocks/src/montgomery/x.rs

@@ -265,13 +265,11 @@ impl ProjectiveMontgomeryXpoint {
     /// Double this point
     // https://eprint.iacr.org/2020/1338.pdf (2.2)
     pub fn double(&self) -> Self {
-        const C: FieldElement = FieldElement(ConstMontyType::new(&U448::from_u64(39082)));
-
         let v1 = (self.U + self.W).square();
         let v2 = (self.U - self.W).square();
         let U = v1 * v2;
         let v3 = v1 - v2;
-        let v4 = C * v3;
+        let v4 = FieldElement::A_PLUS_TWO_OVER_FOUR * v3;
         let v5 = v2 + v4;
         let W = v3 * v5;