feat(nix-compat/narinfo/signature): add new() constructor

This is useful when creating a new Signature struct where the individual
elements are already parsed.

Change-Id: Ie33c66287641951e7a030aaa1e7ff0a86b2628ac
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10111
Reviewed-by: edef <edef@edef.eu>
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/nix-compat/src/narinfo/signature.rs

@@ -4,11 +4,16 @@ use data_encoding::BASE64;
 
 #[derive(Debug)]
 pub struct Signature<'a> {
+    /// TODO(edef): be stricter with signature names here, they especially shouldn't have newlines!
     name: &'a str,
     bytes: [u8; 64],
 }
 
 impl<'a> Signature<'a> {
+    pub fn new(name: &'a str, bytes: [u8; 64]) -> Self {
+        Self { name, bytes }
+    }
+
     pub fn parse(input: &'a str) -> Result<Signature<'a>, SignatureError> {
         let (name, bytes64) = input
             .split_once(':')