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/builtins/mod.rs

@@ -343,6 +343,17 @@ mod pure_builtins {
         serde_json::from_str(&json_str).map_err(|err| err.into())
     }
 
+    #[builtin("toJSON")]
+    fn builtin_to_json(vm: &mut VM, val: Value) -> Result<Value, ErrorKind> {
+        // All thunks need to be evaluated before serialising, as the
+        // data structure is fully traversed by the Serializer (which
+        // does not have a `VM` available).
+        val.deep_force(vm, &mut Default::default())?;
+
+        let json_str = serde_json::to_string(&val)?;
+        Ok(json_str.into())
+    }
+
     #[builtin("genericClosure")]
     fn builtin_generic_closure(vm: &mut VM, input: Value) -> Result<Value, ErrorKind> {
         let attrs = input.to_attrs()?;