Acid protection update

* Adds support for body_parts_covered flags protecting body parts from acid, This means if your only source of protection is your suit (for example) then it will degrade much quicker due to having to protect more than just the chest slot now. this is both a positive and a negative, the positive being that if it looks like it protects your arms, it does.

* Rewrites the code blocks to be neater.

* Item acid_act() grey good pixel_x and _y changed

* Inventory items worn internally (inside armour) only melt when the armour covering them is melted, exclusions are Back items, Belt items, and items held in the hands, since those are visibly external

* Makes item acid_act() check that the Turf exists first.

if you can't understand it from Git's diff:
It's the same hierarchical structure as before, armour closer to the body is checked after armour further away, eg: Suit --> uniform --> painful acidy death. but with body_parts_covered flags counting.