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/function.rs

@@ -39,7 +39,7 @@ impl Formals {
 /// OpThunkSuspended referencing it.  At runtime `Lambda` is usually wrapped
 /// in `Rc` to avoid copying the `Chunk` it holds (which can be
 /// quite large).
-#[derive(Debug, Default, PartialEq)]
+#[derive(Debug, Default)]
 pub struct Lambda {
     pub(crate) chunk: Chunk,
 
@@ -62,7 +62,7 @@ impl Lambda {
     }
 }
 
-#[derive(Clone, Debug, PartialEq)]
+#[derive(Clone, Debug)]
 pub struct Closure {
     pub lambda: Rc<Lambda>,
     pub upvalues: Upvalues,