[READY] 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 so people can discuss them to their heart's content (That said mods could still make handed items if they desired to once this goes in).

Edit: Once I realized my stupid mistake all the error messages could be fixed (as well as with Kevin's helpful advice for the others). This can be merged with no ill-effects now.
Whatever else is happening, this (or whatever writes it to the artifact file) isn't doing the job; as Blaze reported, there's an extra quotation mark getting added, and that breaks savegames.