Unify identity treatment across both backends

src/ark_curve/element/projective.rs

@@ -20,6 +20,10 @@ impl Element {
     pub const GENERATOR: Self = Self {
         inner: EdwardsProjective::new_unchecked(B_X, B_Y, B_T, B_Z),
     };
+
+    pub const IDENTITY: Self = Self {
+        inner: EdwardsProjective::new_unchecked(Fq::ZERO, Fq::ONE, Fq::ZERO, Fq::ONE),
+    };
 }
 
 impl Hash for Element {

src/min_curve/element.rs

@@ -108,6 +108,13 @@ impl Element {
             Element { x, y, z, t }
         }
     }
+    ///
+    /// Convenience method to make identity checks more readable.
+    pub fn is_identity(&self) -> bool {
+        // Section 4.5 of Decaf paper states for cofactor 4 curves we can
+        // just check X = 0 to check equality with identity
+        self.x == Fq::ZERO
+    }
 
     pub fn double(self) -> Self {
         // https://eprint.iacr.org/2008/522 Section 3.3