Restore patches/dalek-ff-group

Ensures `crypto/dalek-ff-group` is pure.

patches/dalek-ff-group/src/lib.rs

@@ -0,0 +1,36 @@
+#![allow(deprecated)]
+#![cfg_attr(docsrs, feature(doc_cfg))]
+#![no_std] // Prevents writing new code, in what should be a simple wrapper, which requires std
+#![doc = include_str!("../README.md")]
+#![allow(clippy::redundant_closure_call)]
+
+pub use dalek_ff_group::{Scalar, EdwardsPoint, RistrettoPoint, Ed25519, Ristretto};
+
+type ThirtyTwoArray = [u8; 32];
+prime_field::odd_prime_field_with_specific_repr!(
+  FieldElement,
+  "0x7fffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffed",
+  "02",
+  false,
+  crate::ThirtyTwoArray
+);
+
+impl FieldElement {
+  /// Create a FieldElement from a `crypto_bigint::U256`.
+  ///
+  /// This will reduce the `U256` by the modulus, into a member of the field.
+  #[deprecated]
+  pub const fn from_u256(u256: &crypto_bigint_05::U256) -> Self {
+    const MODULUS: crypto_bigint::U256 = crypto_bigint::U256::from_be_hex(
+      "7fffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffed",
+    );
+    let mut u256 = crypto_bigint::U256::from_words(*u256.as_words());
+    loop {
+      let result = FieldElement::from_bytes(&u256.to_le_bytes());
+      if let Some(result) = result {
+        return result;
+      }
+      u256 = u256.wrapping_sub(&MODULUS);
+    }
+  }
+}