refactor(tvix/nix-compat): make NixHash an enum with fixed-len bytes

Less Vec<u8> passed around.

Change-Id: Ie153a6bfaa084d7490ffa38634efdf5f3c31a768
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9722
Reviewed-by: Connor Brewster <cbrewster@hey.com>
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/nix-compat/src/nixhash/algos.rs

@@ -13,6 +13,18 @@ pub enum HashAlgo {
     Sha512,
 }
 
+impl HashAlgo {
+    // return the number of bytes in the digest of the given hash algo.
+    pub fn digest_length(&self) -> usize {
+        match self {
+            HashAlgo::Sha1 => 20,
+            HashAlgo::Sha256 => 32,
+            HashAlgo::Sha512 => 64,
+            HashAlgo::Md5 => 16,
+        }
+    }
+}
+
 impl Display for HashAlgo {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         match &self {