Fix field_offset! to avoid ptr->int casts

• Fix field_offset! macro to avoid ptr -> int casts
  • These have been removed in recent nightlies,
    since pointers can't nessicarrily have a meaningful integer values.
  • I beleive this is because of issues with pointer 'provenance'.
    Here is a good blog post explaining the issue in depth: https://www.ralfj.de/blog/2020/12/14/provenance.html