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/builtins/mod.rs

@@ -250,7 +250,7 @@ mod pure_builtins {
     #[builtin("elem")]
     fn builtin_elem(vm: &mut VM, x: Value, xs: Value) -> Result<Value, ErrorKind> {
         for val in xs.to_list()? {
-            if val.nix_eq(&x, vm)? {
+            if vm.nix_eq(val, x.clone(), true)? {
                 return Ok(true.into());
             }
         }