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/fields/fq.rs

@@ -0,0 +1,7 @@
+use ark_ff::fields::{Fp256, MontBackend, MontConfig};
+
+#[derive(MontConfig)]
+#[modulus = "57896044618658097711785492504343953926634992332820282019728792003956564819949"]
+#[generator = "2"]
+pub struct FqConfig;
+pub type Fq = Fp256<MontBackend<FqConfig, 4>>;