Add prime-field crate

prime-field introduces a macro to generate a prime field, in its entitrety,
de-duplicating code across minimal-ed448, embedwards25519, and secq256k1.

crypto/ed448/src/lib.rs

@@ -1,19 +1,47 @@
 #![cfg_attr(docsrs, feature(doc_auto_cfg))]
 #![doc = include_str!("../README.md")]
 #![no_std]
-#![allow(clippy::redundant_closure_call)]
 
-#[macro_use]
-mod backend;
+prime_field::odd_prime_field!(
+  FieldElement,
+  // 2**448 - 2**224 - 1
+  concat!(
+    "00",
+    "fffffffffffffffffffffffffffffffffffffffffffffffffffffffe",
+    "ffffffffffffffffffffffffffffffffffffffffffffffffffffffff",
+  ),
+  "07",
+  false
+);
 
-mod scalar;
-pub use scalar::Scalar;
-
-mod field;
-pub use field::FieldElement;
+prime_field::odd_prime_field!(
+  Scalar,
+  concat!(
+    "00",
+    "3fffffffffffffffffffffffffffffffffffffffffffffffffffffff",
+    "7cca23e9c44edb49aed63690216cc2728dc58f552378c292ab5844f3",
+  ),
+  "02",
+  false
+);
 
 mod point;
 pub use point::Point;
 
 mod ciphersuite;
 pub use crate::ciphersuite::Ed448;
+
+pub(crate) fn u8_from_bool(bit_ref: &mut bool) -> u8 {
+  use core::hint::black_box;
+  use prime_field::zeroize::Zeroize;
+
+  let bit_ref = black_box(bit_ref);
+
+  let mut bit = black_box(*bit_ref);
+  let res = black_box(u8::from(bit));
+  bit.zeroize();
+  debug_assert!((res | 1) == 1);
+
+  bit_ref.zeroize();
+  res
+}