Add the curve25519 curve (#124)

* add ed25519 curve

* changelog

* curve info

* fix

* edit

* update

* add more explanation

* Update curve25519/src/curves/mod.rs

* Update curve25519/src/curves/mod.rs

* fixed the comment

* Update curve25519/src/curves/mod.rs

Co-authored-by: onewayfunc <onewayfunc@gmail.com>

curve25519/src/constraints/fields.rs

@@ -0,0 +1,11 @@
+use ark_r1cs_std::fields::fp::FpVar;
+
+use crate::Fq;
+
+/// A variable that is the R1CS equivalent of `crate::Fq`.
+pub type FqVar = FpVar<Fq>;
+
+#[test]
+fn test() {
+    ark_curve_constraint_tests::fields::field_test::<_, _, FqVar>().unwrap();
+}