chore: Move hashing functions into sodium/ciphers crate

This finishes the last step of removing sodium.rs from the rosenpass crate
itself and also removes the NOTHING and NONCE0 constants.

Hashing functions now use destination parameters;
rosenpass_constant_time::xor now does too.

constant-time/Cargo.toml

@@ -12,3 +12,4 @@ readme = "readme.md"
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
+rosenpass-to = { path = "../to" }

constant-time/src/lib.rs

@@ -1,18 +1,26 @@
-/// Xors a and b element-wise and writes the result into a.
+use rosenpass_to::{with_destination, To};
+
+/// Xors the source into the destination
 ///
 /// # Examples
 ///
 /// ```
-/// use rosenpass_constant_time::xor_into;
-/// let mut a = String::from("hello").into_bytes();
-/// let b = b"world";
-/// xor_into(&mut a, b);
-/// assert_eq!(&a, b"\x1f\n\x1e\x00\x0b");
+/// use rosenpass_constant_time::xor;
+/// use rosenpass_to::To;
+/// assert_eq!(
+///     xor(b"world").to_this(|| b"hello".to_vec()),
+///     b"\x1f\n\x1e\x00\x0b");
 /// ```
+///
+/// # Panics
+///
+/// If source and destination are of different sizes.
 #[inline]
-pub fn xor_into(a: &mut [u8], b: &[u8]) {
-    assert!(a.len() == b.len());
-    for (av, bv) in a.iter_mut().zip(b.iter()) {
-        *av ^= *bv;
-    }
+pub fn xor<'a>(src: &'a [u8]) -> impl To<[u8], ()> + 'a {
+    with_destination(|dst: &mut [u8]| {
+        assert!(src.len() == dst.len());
+        for (dv, sv) in dst.iter_mut().zip(src.iter()) {
+            *dv ^= *sv;
+        }
+    })
 }