feat(tvix/eval): Implement builtins.deepSeq

This is done via a new `deepForce` function on Value. Since values can
be cyclical (for example, see the test-case), we need to do some extra
work to avoid RefCell borrow errors if we ever hit a graph cycle:

While deep-forcing values, we keep a set of thunks that we have
already seen and avoid doing any work on the same thunk twice. The set
is encapsulated in a separate type to stop potentially invalid
pointers from leaking out.

Finally, since deep_force is conceptually similar to
`VM::force_for_output` (but more suited to usage in eval since it
doesn't clone the values) this removes the latter, replacing it with
the former.

Co-Authored-By: Vincent Ambo <tazjin@tvl.su>
Change-Id: Iefddefcf09fae3b6a4d161a5873febcff54b9157
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7000
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/eval/src/value/list.rs

@@ -102,3 +102,13 @@ impl IntoIterator for NixList {
         self.0.into_iter()
     }
 }
+
+impl<'a> IntoIterator for &'a NixList {
+    type Item = &'a Value;
+
+    type IntoIter = std::slice::Iter<'a, Value>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        self.0.iter()
+    }
+}