tvix/eval: suspended native thunks use fake spans

The implementation of suspended native thunks currently uses a "fake" span, with a codemap that is not the same as the global codemap.

This means that if a suspended native thunk returns an error, or emits a warning, tvix will crash as it attempts to look up the fake span in the actual codemap, which will panic.

We need to thread through a real span, or even the real codemap, in some way.

1. tazjin closed this issue at 2023-03-14T09·19+00