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/lib.rs

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