[WIP] Inhand overhaul

Credits to Wjohnston for the dmi editing, I just did the dm changes.

Basically, all the inhands and equipped overlays were moved from the big monolithic dmis to each individual object dmi, appended by _l, _r, _back, _belt, etc. The update_icon code has been changed to account for this. Furthermore, icon_state is now the main choice for inhand names, with item_state (along with the new item_state_icon) persisting as a fallback.

This is marked as WIP because the new proc in living/update_icon.dm performs too many icon operations and lags a lot at roundstart. It does cache so it's not the end of the world, but at the beginning it takes a while to catch up. All help is appreciated.