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/value/mod.rs

@@ -78,7 +78,7 @@ pub enum Value {
     #[serde(skip)]
     UnresolvedPath(Box<Path>),
     #[serde(skip)]
-    Json(serde_json::Value),
+    Json(Box<serde_json::Value>),
 
     #[serde(skip)]
     FinaliseRequest(bool),