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/errors.rs

@@ -213,7 +213,7 @@ impl ErrorKind {
 impl From<serde_json::Error> for ErrorKind {
     fn from(err: serde_json::Error) -> Self {
         // Can't just put the `serde_json::Error` in the ErrorKind since it doesn't impl `Clone`
-        Self::FromJsonError(format!("Error parsing JSON: {err}"))
+        Self::FromJsonError(format!("error in JSON serialization: {err}"))
     }
 }