refactor(nix-compat): Properly encapsulate store path construction

Before there was code scattered about (e.g. text hashing module and
derivation output computation) constructing store paths from low level
building blocks --- there was some duplication and it was easy to make
nonsense store paths.

Now, we have roughly the same "safe-ish" ways of constructing them as
C++ Nix, and only those are exposed:

- Make text hashed content-addressed store paths

- Make other content-addressed store paths

- Make input-addressed fixed output hashes

Change-Id: I122a3ee0802b4f45ae386306b95b698991be89c8
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8411
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

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

@@ -5,7 +5,7 @@ use serde::{Deserialize, Serialize};
 use crate::nixhash::Error;
 
 /// This are the hash algorithms supported by cppnix.
-#[derive(Clone, Debug, Eq, PartialEq, Serialize, Deserialize)]
+#[derive(Clone, Copy, Debug, Eq, PartialEq, Serialize, Deserialize)]
 pub enum HashAlgo {
     Md5,
     Sha1,