Elevates (almost all) inventory variables from /carbon to /human (#96955)

## About The Pull Request

Moves all inventory slots but handcuffs/legcuffs (as those can be used
by xenos) from ``/carbon`` onto ``/human``, as xenos do not use any of
those inventory slots, only leaving them in use by humans. Their
presence on ``/carbon`` is an artifact of times when monkeys weren't
humans, and some of the slots had to be shared by all carbons.
In some places I've used ``get_item_by_slot`` rather than swapping
checks to ``ishuman`` for simplicity's sake, in some places it might not
be the most optimal solution but in cases like help act any other
solution would require a refactor of the whole (massive) proc.

## Why It's Good For The Game

Cleaner/more sensible code, one step closer to fully datumized
inventories.

## Changelog
🆑
refactor: Moved a lot of human-specific inventory code onto human mobs,
report if inventories break!
/🆑

code/modules/reagents/chemistry/reagents/toxin_reagents.dm

@@ -496,12 +496,12 @@
 		if(exposed_mob.adjust_tox_loss(damage * 20, required_biotype = affected_biotype))
 			return
 
-	if(!(methods & VAPOR) || !iscarbon(exposed_mob))
+	if(!(methods & VAPOR) || !ishuman(exposed_mob))
 		return
 
-	var/mob/living/carbon/exposed_carbon = exposed_mob
-	if(!exposed_carbon.wear_mask)
-		exposed_carbon.adjust_tox_loss(damage, required_biotype = affected_biotype)
+	var/mob/living/carbon/human/exposed_human = exposed_mob
+	if(!exposed_human.wear_mask)
+		exposed_human.adjust_tox_loss(damage, required_biotype = affected_biotype)
 
 /datum/reagent/toxin/plantbgone/weedkiller
 	name = "Weed Killer"