refactor(tvix/eval): Box the inside of Value::Json

serde_json::Value is pretty large, and is contributing (albeit not
exclusively) to the large size of the Value repr. Putting it in a box
is *especially* cheap (since it's rarely used) and allows us
to (eventually) cut down on the size of Value.

Change-Id: I005a802d8527b639beb4e938e3320b11ffa1ef23
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10795
Reviewed-by: sterni <sternenseemann@systemli.org>
Autosubmit: aspen <root@gws.fyi>
Tested-by: BuildkiteCI

tvix/eval/src/vm/generators.rs

@@ -775,7 +775,7 @@ pub(crate) async fn request_to_json(
     value: Value,
 ) -> Result<serde_json::Value, CatchableErrorKind> {
     match co.yield_(VMRequest::ToJson(value)).await {
-        VMResponse::Value(Value::Json(json)) => Ok(json),
+        VMResponse::Value(Value::Json(json)) => Ok(*json),
         VMResponse::Value(Value::Catchable(cek)) => Err(cek),
         msg => panic!(
             "Tvix bug: VM responded with incorrect generator message: {}",