feat(tvix/eval): non-recursive implementation of nix_eq()

This passes all the function/thunk-pointer-equality tests in
cl/7369.

Change-Id: Ib47535ba2fc77a4f1c2cc2fd23d3a879e21d8b4c
Signed-off-by: Adam Joseph <adam@westernsemico.com>
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7358
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/eval/src/value/thunk.rs

@@ -190,6 +190,12 @@ impl Thunk {
             thunk => panic!("upvalues() on non-suspended thunk: {thunk:?}"),
         })
     }
+
+    /// Do not use this without first reading and understanding
+    /// `tvix/docs/value-pointer-equality.md`.
+    pub(crate) fn ptr_eq(&self, other: &Self) -> bool {
+        Rc::ptr_eq(&self.0, &other.0)
+    }
 }
 
 impl TotalDisplay for Thunk {