feat(tvix/eval): add SharedThunkSet

This is a ThunkSet wrapped to be shareable, which will be required
once ThunkSets are embedded in futures.

Change-Id: I5a067b7972ac86e4d354c75ef05c86b2284c1137
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8144
Tested-by: BuildkiteCI
Reviewed-by: raitobezarius <tvl@lahfa.xyz>

tvix/eval/src/value/thunk.rs

@@ -488,3 +488,14 @@ impl ThunkSet {
         self.0.insert(ptr)
     }
 }
+
+#[derive(Default, Clone)]
+pub struct SharedThunkSet(Rc<RefCell<ThunkSet>>);
+
+impl SharedThunkSet {
+    /// Check whether the given thunk has already been seen. Will mark the thunk
+    /// as seen otherwise.
+    pub fn insert(&self, thunk: &Thunk) -> bool {
+        self.0.borrow_mut().insert(thunk)
+    }
+}