Custom items revamp DONUT MERGE UNTESTED

no more bloody ckey checks in my beautiful subsystem
only parses once, instead of every time someone spawns
equips custom items to the appropriate slot if it can (probably unlikely, given that it's done after everything for the job is equipped) before falling back to backpack/box/ground

people who will NEED config changes (can be done on live now without breaking anything):
jakksergal: Nashi Ra'hal: /obj/item/weapon/storage/belt/medical/fluff/nashi_belt: Nurse, Chemist

other ones can be simplified by this (ie removing a bunch of the definitions) but will continue working fine (even if they are updated before this hits master)
Easy enough to change later if it does end up being an awful idea, too. (item_spawning.dm:54, change it to just var/CurrentTitle = M.mind.assigned_role)