Start experimenting with full generation of all of the numeric types.

Previously, we used a little bit of generation to drive a lot of Rust
macros. This works, but it's a little confusing to read and write. In
addition, we used a lot of implementations with variable timings based
on their input, which isn't great for crypto. This is the start of an
attempt to just generate all of the relevant Rust code directly, and to
use timing-channel resistant implementations for most of the routines.

old/unsigned/sub.rs

@@ -0,0 +1,107 @@
+use unsigned::add::unsafe_addition;
+
+pub fn subtract(res: &mut [u64], spare: &mut [u64], other: &mut [u64])
+{
+    for i in 0..res.len() {
+        other[i] = !other[i];
+    }
+    spare[0] = 1;
+    unsafe_addition(other, &spare, true);
+    unsafe_addition(res,   &other, true);
+}
+
+macro_rules! subtraction_impls
+{
+    ($name: ident, $size: expr) => {
+        impl SubAssign for $name {
+            fn sub_assign(&mut self, rhs: $name) {
+                let mut temp = [0; $size];
+                subtract(&mut self.value, &mut temp, &mut rhs.value.clone());
+            }
+        }
+
+        impl<'a> SubAssign<&'a $name> for $name {
+            fn sub_assign(&mut self, rhs: &$name) {
+                let mut temp = [0; $size];
+                subtract(&mut self.value, &mut temp, &mut rhs.value.clone());
+            }
+        }
+
+        impl Sub for $name {
+            type Output = $name;
+
+            fn sub(self, rhs: $name) -> $name {
+                let mut res  = self.clone();
+                let mut temp = [0; $size];
+                subtract(&mut res.value, &mut temp, &mut rhs.value.clone());
+                res
+            }
+        }
+
+        impl<'a> Sub<$name> for &'a $name {
+            type Output = $name;
+
+            fn sub(self, rhs: $name) -> $name {
+                let mut res  = self.clone();
+                let mut temp = [0; $size];
+                subtract(&mut res.value, &mut temp, &mut rhs.value.clone());
+                res
+            }
+        }
+
+        impl<'a> Sub<&'a $name> for $name {
+            type Output = $name;
+
+            fn sub(self, rhs: &$name) -> $name {
+                let mut res  = self.clone();
+                let mut temp = [0; $size];
+                subtract(&mut res.value, &mut temp, &mut rhs.value.clone());
+                res
+            }
+        }
+
+        impl<'a,'b> Sub<&'a $name> for &'b $name {
+            type Output = $name;
+
+            fn sub(self, rhs: &$name) -> $name {
+                let mut res  = self.clone();
+                let mut temp = [0; $size];
+                subtract(&mut res.value, &mut temp, &mut rhs.value.clone());
+                res
+            }
+        }
+    }
+}
+
+#[cfg(test)]
+macro_rules! generate_sub_tests {
+    ($name: ident, $lname: ident) => {
+        #[test]
+        fn $lname() {
+            generate_sub_tests!(body $name, $lname);
+        }
+    };
+    (ignore $name: ident, $lname: ident) => {
+        #[test]
+        #[ignore]
+        fn $lname() {
+            generate_sub_tests!(body $name, $lname);
+        }
+    };
+    (body $name: ident, $lname: ident) => {
+        let fname = build_test_path("sub", stringify!($name));
+        run_test(fname.to_string(), 3, |case| {
+            let (neg0, abytes) = case.get("a").unwrap();
+            let (neg1, bbytes) = case.get("b").unwrap();
+            let (neg2, cbytes) = case.get("c").unwrap();
+            assert!(!neg0 && !neg1 && !neg2);
+
+            let mut a = $name::from_bytes(abytes);
+            let b     = $name::from_bytes(bbytes);
+            let c     = $name::from_bytes(cbytes);
+            assert_eq!(c, &a - &b);
+            a -= b;
+            assert_eq!(c, a);
+        });
+    };
+}