Implement a DLEq library

While Serai only needs the simple DLEq which was already present under 
monero, this migrates the implementation of the cross-group DLEq I 
maintain into Serai. This was to have full access to the ecosystem of 
libraries built under Serai while also ensuring support for it.

The cross_group curve, which is extremely experimental, is feature 
flagged off. So is the built in serialization functionality, as this 
should be possible to make nostd once const generics are full featured, 
yet the implemented serialization adds the additional barrier of 
std::io.

crypto/dleq/src/tests/cross_group/mod.rs

@@ -0,0 +1,54 @@
+mod scalar;
+mod schnorr;
+
+use hex_literal::hex;
+use rand_core::OsRng;
+
+use ff::Field;
+use group::{Group, GroupEncoding};
+
+use k256::{Scalar, ProjectivePoint};
+use dalek_ff_group::{EdwardsPoint, CompressedEdwardsY};
+
+use transcript::RecommendedTranscript;
+
+use crate::{Generators, cross_group::DLEqProof};
+
+#[test]
+fn test_dleq() {
+  let transcript = || RecommendedTranscript::new(b"Cross-Group DLEq Proof Test");
+
+  let generators = (
+    Generators::new(
+      ProjectivePoint::GENERATOR,
+      ProjectivePoint::from_bytes(
+        &(hex!("0250929b74c1a04954b78b4b6035e97a5e078a5a0f28ec96d547bfee9ace803ac0").into())
+      ).unwrap()
+    ),
+
+    Generators::new(
+      EdwardsPoint::generator(),
+      CompressedEdwardsY::new(
+        hex!("8b655970153799af2aeadc9ff1add0ea6c7251d54154cfa92c173a0dd39c1f94")
+      ).decompress().unwrap()
+    )
+  );
+
+  let key = Scalar::random(&mut OsRng);
+  let (proof, keys) = DLEqProof::prove(&mut OsRng, &mut transcript(), generators, key);
+
+  let public_keys = proof.verify(&mut transcript(), generators).unwrap();
+  assert_eq!(generators.0.primary * keys.0, public_keys.0);
+  assert_eq!(generators.1.primary * keys.1, public_keys.1);
+
+  #[cfg(feature = "serialize")]
+  {
+    let mut buf = vec![];
+    proof.serialize(&mut buf).unwrap();
+    let deserialized = DLEqProof::<ProjectivePoint, EdwardsPoint>::deserialize(
+      &mut std::io::Cursor::new(&buf)
+    ).unwrap();
+    assert_eq!(proof, deserialized);
+    deserialized.verify(&mut transcript(), generators).unwrap();
+  }
+}