feat(tvix/eval): implement builtins.toJSON

Implements `Serialize` for `tvix_eval::Value`. Special care is taken
with serialisation of attribute sets, and forcing of thunks.

The tests should cover both cases well.

Change-Id: I9bb135bacf6f87bc6bd0bd88cef0a42308e6c335
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7803
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Autosubmit: tazjin <tazjin@tvl.su>

tvix/eval/src/value/thunk.rs

@@ -24,6 +24,8 @@ use std::{
     rc::Rc,
 };
 
+use serde::Serialize;
+
 use crate::{
     chunk::Chunk,
     errors::{Error, ErrorKind},
@@ -329,6 +331,15 @@ impl TotalDisplay for Thunk {
     }
 }
 
+impl Serialize for Thunk {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: serde::Serializer,
+    {
+        self.value().serialize(serializer)
+    }
+}
+
 /// A wrapper type for tracking which thunks have already been seen in a
 /// context. This is necessary for cycle detection.
 ///