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.

sodium/src/hash/blake2b.rs

@@ -0,0 +1,31 @@
+use libsodium_sys as libsodium;
+use rosenpass_to::{with_destination, To};
+use std::ffi::c_ulonglong;
+use std::ptr::null;
+
+pub const KEY_MIN: usize = libsodium::crypto_generichash_blake2b_KEYBYTES_MIN as usize;
+pub const KEY_MAX: usize = libsodium::crypto_generichash_blake2b_KEYBYTES_MAX as usize;
+pub const OUT_MIN: usize = libsodium::crypto_generichash_blake2b_BYTES_MIN as usize;
+pub const OUT_MAX: usize = libsodium::crypto_generichash_blake2b_BYTES_MAX as usize;
+
+#[inline]
+pub fn hash<'a>(key: &'a [u8], data: &'a [u8]) -> impl To<[u8], anyhow::Result<()>> + 'a {
+    with_destination(|out: &mut [u8]| {
+        assert!(key.is_empty() || (KEY_MIN <= key.len() && key.len() <= KEY_MAX));
+        assert!(OUT_MIN <= out.len() && out.len() <= OUT_MAX);
+        let kptr = match key.len() {
+            // NULL key
+            0 => null(),
+            _ => key.as_ptr(),
+        };
+        sodium_call!(
+            crypto_generichash_blake2b,
+            out.as_mut_ptr(),
+            out.len(),
+            data.as_ptr(),
+            data.len() as c_ulonglong,
+            kptr,
+            key.len()
+        )
+    })
+}

sodium/src/hash/mod.rs

@@ -0,0 +1 @@
+pub mod blake2b;

sodium/src/lib.rs

@@ -16,4 +16,5 @@ pub fn init() -> anyhow::Result<()> {
 }
 
 pub mod aead;
+pub mod hash;
 pub mod helpers;