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/cross_group/scalar.rs

@@ -0,0 +1,34 @@
+use ff::PrimeFieldBits;
+
+/// Convert a uniform scalar into one usable on both fields, clearing the top bits as needed
+pub fn scalar_normalize<F0: PrimeFieldBits, F1: PrimeFieldBits>(scalar: F0) -> (F0, F1) {
+  let mutual_capacity = F0::CAPACITY.min(F1::CAPACITY);
+
+  // The security of a mutual key is the security of the lower field. Accordingly, this bans a
+  // difference of more than 4 bits
+  #[cfg(feature = "secure_capacity_difference")]
+  assert!((F0::CAPACITY.max(F1::CAPACITY) - mutual_capacity) < 4);
+
+  let mut res1 = F0::zero();
+  let mut res2 = F1::zero();
+  // Uses the bit view API to ensure a consistent endianess
+  let mut bits = scalar.to_le_bits();
+  // Convert it to big endian
+  bits.reverse();
+  for bit in bits.iter().skip(bits.len() - usize::try_from(mutual_capacity).unwrap()) {
+    res1 = res1.double();
+    res2 = res2.double();
+    if *bit {
+      res1 += F0::one();
+      res2 += F1::one();
+    }
+  }
+
+  (res1, res2)
+}
+
+/// Helper to convert a scalar between fields. Returns None if the scalar isn't mutually valid
+pub fn scalar_convert<F0: PrimeFieldBits, F1: PrimeFieldBits>(scalar: F0) -> Option<F1> {
+  let (valid, converted) = scalar_normalize(scalar);
+  Some(converted).filter(|_| scalar == valid)
+}