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/codec.rs

@@ -0,0 +1,90 @@
+/// Conversion from bytes into the numeric type.
+pub trait Decoder {
+    fn from_bytes(x: &[u8]) -> Self;
+}
+
+/// Conversion from the numeric types into a byte buffer.
+pub trait Encoder {
+    fn to_bytes(&self) -> Vec<u8>;
+}
+
+pub(crate) fn raw_decoder(input: &[u8], output: &mut [u64])
+{
+    let mut item = 0;
+    let mut shift = 0;
+    let mut idx = 0;
+
+    for v in input.iter().rev() {
+        item |= (*v as u64) << shift;
+        shift += 8;
+        if shift == 64 {
+            shift = 0;
+            output[idx] = item;
+            idx += 1;
+            item = 0;
+        }
+    }
+    if item != 0 {
+        output[idx] = item;
+    }
+}
+
+macro_rules! generate_decoder {
+    ($name: ident) => {
+        impl Decoder for $name {
+            fn from_bytes(x: &[u8]) -> $name {
+                let mut res = $name::zero();
+                raw_decoder(x, &mut res.value);
+                res
+            }
+        }
+    }
+}
+
+macro_rules! generate_encoder {
+    ($name: ident) => {
+        impl Encoder for $name {
+            fn to_bytes(&self) -> Vec<u8> {
+                let mut res = Vec::with_capacity(self.value.len() * 8);
+                for v in self.value.iter().rev() {
+                    let val = *v;
+                    res.push( (val >> 56) as u8);
+                    res.push( (val >> 48) as u8);
+                    res.push( (val >> 40) as u8);
+                    res.push( (val >> 32) as u8);
+                    res.push( (val >> 24) as u8);
+                    res.push( (val >> 16) as u8);
+                    res.push( (val >>  8) as u8);
+                    res.push( (val >>  0) as u8);
+                }
+                res
+            }
+        }
+    }
+}
+
+macro_rules! generate_codec
+{
+    ($name: ident) => {
+        generate_decoder!($name);
+        generate_encoder!($name);
+    }
+}
+
+#[cfg(test)]
+macro_rules! generate_codec_tests {
+    ($name: ident, $lname: ident) => {
+        #[cfg(test)]
+        mod $lname {
+            use super::super::super::*;
+
+            quickcheck! {
+                fn decode_encode(x: $name) -> bool {
+                    let bytes = x.to_bytes();
+                    let x2    = $name::from_bytes(&bytes);
+                    x == x2
+                }
+            }
+        }
+    };
+}