Implement Default for remaining point types.

* FIXES https://github.com/dalek-cryptography/curve25519-dalek/issues/154

src/edwards.rs

@@ -257,6 +257,12 @@ impl Identity for CompressedEdwardsY {
     }
 }
 
+impl Default for CompressedEdwardsY {
+    fn default() -> CompressedEdwardsY {
+        CompressedEdwardsY::identity()
+    }
+}
+
 impl Identity for EdwardsPoint {
     fn identity() -> EdwardsPoint {
         EdwardsPoint{ X: FieldElement::zero(),
@@ -266,6 +272,12 @@ impl Identity for EdwardsPoint {
     }
 }
 
+impl Default for EdwardsPoint {
+    fn default() -> EdwardsPoint {
+        EdwardsPoint::identity()
+    }
+}
+
 // ------------------------------------------------------------------------
 // Validity checks (for debugging, not CT)
 // ------------------------------------------------------------------------

src/montgomery.rs

@@ -155,6 +155,12 @@ impl Identity for ProjectivePoint {
     }
 }
 
+impl Default for ProjectivePoint {
+    fn default() -> ProjectivePoint {
+        ProjectivePoint::identity()
+    }
+}
+
 impl ConditionallyAssignable for ProjectivePoint {
     fn conditional_assign(&mut self, that: &ProjectivePoint, choice: Choice) {
         self.U.conditional_assign(&that.U, choice);

src/ristretto.rs

@@ -279,6 +279,12 @@ impl Identity for CompressedRistretto {
     }
 }
 
+impl Default for CompressedRistretto {
+    fn default() -> CompressedRistretto {
+        CompressedRistretto::identity()
+    }
+}
+
 // ------------------------------------------------------------------------
 // Serde support
 // ------------------------------------------------------------------------
@@ -661,6 +667,12 @@ impl Identity for RistrettoPoint {
     }
 }
 
+impl Default for RistrettoPoint {
+    fn default() -> RistrettoPoint {
+        RistrettoPoint::identity()
+    }
+}
+
 // ------------------------------------------------------------------------
 // Equality
 // ------------------------------------------------------------------------