- Moves as much data out of the `Vm`
- Removes duplication with call_vm and call_vt flavours
- Uses tracked chain instead of fixed int for determining max nesting depth
- This means that nesting checks now generalizes to layout and realization, to detect crashing show rules and overly nested layouts
Things like `luma(1, key: "val")` didn't produce an error before because `args.finish()?` wasn't called. This changes `args: Args` to `args: &mut Args` to make it impossible for that to happen.
