Improve performance of IncompletePoint addition

We only need to track the occurrence of any edge cases, and we can do so
without expensive inversions at every addition step, by instead
performing the checks on the projective form directly.

src/primitives/sinsemilla/addition.rs

@@ -1,7 +1,7 @@
 use std::ops::Add;
 
-use group::Curve;
-use pasta_curves::{arithmetic::CurveAffine, pallas};
+use group::Group;
+use pasta_curves::pallas;
 use subtle::{ConstantTimeEq, CtOption};
 
 /// P ∪ {⊥}
@@ -33,14 +33,13 @@ impl Add for IncompletePoint {
             rhs.0.and_then(|q| {
                 // 0 ⊹ 0 = ⊥
                 // 0 ⊹ P = ⊥
-                p.to_affine().coordinates().and_then(|c_p| {
-                    // P ⊹ 0 = ⊥
-                    q.to_affine().coordinates().and_then(|c_q| {
-                        // (x, y) ⊹ (x', y') = ⊥ if x == x'
-                        // (x, y) ⊹ (x', y') = (x, y) + (x', y') if x != x'
-                        CtOption::new(p + q, !c_p.x().ct_eq(c_q.x()))
-                    })
-                })
+                // P ⊹ 0 = ⊥
+                // (x, y) ⊹ (x', y') = ⊥ if x == x'
+                // (x, y) ⊹ (x', y') = (x, y) + (x', y') if x != x'
+                CtOption::new(
+                    p + q,
+                    !(p.is_identity() | q.is_identity() | p.ct_eq(&q) | p.ct_eq(&-q)),
+                )
             })
         }))
     }