ristretto255: add NewElement and NewScalar

ristretto255.go

@@ -36,13 +36,15 @@ var (
 )
 
 // Element is an element of the ristretto255 prime-order group.
-//
-// The zero value of Element is not valid, but can be used as the receiver for
-// any setting operation.
 type Element struct {
 	r edwards25519.ProjP3
 }
 
+// NewElement returns a new Element set to the identity value.
+func NewElement() *Element {
+	return (&Element{}).Zero()
+}
+
 // Equal returns 1 if e is equivalent to ee, and 0 otherwise.
 //
 // Note that Elements must not be compared in any other way.

scalar.go

@@ -16,6 +16,11 @@ type Scalar struct {
 	s scalar.Scalar
 }
 
+// NewScalar returns a Scalar set to the value 0.
+func NewScalar() *Scalar {
+	return (&Scalar{}).Zero()
+}
+
 // Add sets s = x + y mod l and returns s.
 func (s *Scalar) Add(x, y *Scalar) *Scalar {
 	s.s.Add(&x.s, &y.s)