fix(tvix/eval): remove impl PartialEq for Value

It isn't possible to implement PartialEq properly for Value, because
any sensible implementation needs to force() thunks, which cannot be
done without a `&mut VM`.

The existing derive(PartialEq) has false negatives, which caused the
bug which cl/7142 fixed.  Fortunately that bug was easy to find, but
a silent false negative deep within the bowels of nixpkgs could be a
real nightmare to hunt down.

Let's just remove the PartialEq impl for Value, and the other
derive(PartialEq)'s that depend on it.

Signed-off-by: Adam Joseph <adam@westernsemico.com>
Change-Id: Iacd3726fefc7fc1edadcd7e9b586e04cf8466775
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7144
Reviewed-by: kanepyork <rikingcoding@gmail.com>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/eval/src/value/mod.rs

@@ -31,7 +31,7 @@ pub use thunk::Thunk;
 use self::thunk::ThunkSet;
 
 #[warn(variant_size_differences)]
-#[derive(Clone, Debug, PartialEq)]
+#[derive(Clone, Debug)]
 pub enum Value {
     Null,
     Bool(bool),