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/thunk.rs

@@ -41,7 +41,7 @@ use super::{Lambda, TotalDisplay};
 /// Upvalues must be finalised before leaving the initial state
 /// (Suspended or RecursiveClosure).  The [`value()`] function may
 /// not be called until the thunk is in the final state (Evaluated).
-#[derive(Clone, Debug, PartialEq)]
+#[derive(Clone, Debug)]
 enum ThunkRepr {
     /// Thunk is closed over some values, suspended and awaiting
     /// execution.
@@ -63,7 +63,7 @@ enum ThunkRepr {
 /// evaluation due to self-reference or lazy semantics (or both).
 /// Every reference cycle involving `Value`s will contain at least
 /// one `Thunk`.
-#[derive(Clone, Debug, PartialEq)]
+#[derive(Clone, Debug)]
 pub struct Thunk(Rc<RefCell<ThunkRepr>>);
 
 impl Thunk {