DNA fixes - Update Icons refinements

Commit1:
>Resolves some dna runtimes. - check_dna_integrity() was not being us… … 
…ed properly.

>Removed a for(obj/item in world) loop triggered by bloodying items. It now uses a caching system.
>New skin_tones - they are now sprite-based. This is to prepare for expansion of the mutantrace system. Thanks validsalid x
>Removed fat icons from the game - when you are obese you will now just look like a regular human. You will not be immune to being pushed. You will not be immune to choking. You will still overheat and run slower and appear fat upon "examine" verb use.

Commit2:
 update_icons.dm changes for humans. This ties in with the dna changes… … 
… and the changes to skin_tone and hair colours

>update_mutantrace() is no more, it is now part of update_body()
>Moved a lot of stuff into update_base_icon_state(), this will allow making mutantraces modular.
Almost all icon operations are now removed from human overlays (obviously some are still required for recolouring hair and eyes). Any remaining dynamically generated overlay icons are applied individually. This will allow byond to cache them better. This will also eliminate the bug where humans are initially invisible/unclickable when you first encounter them (due to network latency where the icons are being downloaded).
>UpdateDamageIcons() renamed to update_damage_overlays(), so it has the same sort of name as the other procs
>update_icons=true or false arguments removed from human overaly-update procs, no longer required. (i.e. the update_body() update_inv_whatever etc procs do not call update_icons() - see documentation for details)
>mutantrace icon_states moved into human.dmi
>skeletons are now a mutantrace, not a mutation.
>update_icons system should now be even more efficient (thanks partly to aranclanos)
>documentation rewritten
>copypasted code moved into helper procs - super tidy code :3

Other:
>Removed compile error where the core code was reliant on compiling away mission code. >:[
>s_tone renamed to skin_tone