fix(tvix/eval): thread Display & PartialEq through to thunk values

If a thunk is already evaluated, there are cases where due to the
memoisation implementation something might observe a value wrapped in
a thunk.

In these cases, the implementation of `Display` and `PartialEq` must
delegate to the underlying value.

Note that there are a handful of other cases like these which we need
to cover.

It is a little tricky to write integration tests for these directly,
especially as some of the open-upvalue optimisations coming down the
pipe will reduce the number of observable thunks.

One test that covers a part of this behaviour is currently
disabled (needs some more machinery), but it's being brought back in
the next commits.

Change-Id: Iaa8cd338c12236af844bbc99d8cec2205f0d0095
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6370
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

tvix/eval/src/value/thunk.rs

@@ -20,6 +20,7 @@
 
 use std::{
     cell::{Ref, RefCell, RefMut},
+    fmt::Display,
     rc::Rc,
 };
 
@@ -131,3 +132,12 @@ impl UpvalueCarrier for Thunk {
         })
     }
 }
+
+impl Display for Thunk {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match &*self.0.borrow() {
+            ThunkRepr::Evaluated(v) => v.fmt(f),
+            _ => f.write_str("internal[thunk]"),
+        }
+    }
+}