Merge side attribute into the body framework (framework PR)

Merges the side attribute into the body_part framework. This doesn't include any of the JSON changes, which will be coming in a later PR. I didn't remove the errors messages because other, hidden errors require the player to load->save->reload old saves to fix them. As such the game still produces 3 harmless error messages on the first load of old saves, which we can use to remind players that they need to save/reload old saves for them to work correctly.