reagent holder rework (#6903)

- reagents are now singletons
- most of reagent holder datums are refactored.

code/game/objects/structures/medical_stand.dm

@@ -423,15 +423,10 @@
 				return
 
 			// If the human is losing too much blood, beep.
-			if(H.vessel.get_reagent_amount("blood") < H.species.blood_volume*H.species.blood_level_safe)
+			if(H.blood_holder.get_total_volume() < H.species.blood_volume*H.species.blood_level_safe)
 				visible_message("\The [src] beeps loudly.")
 
-			var/datum/reagent/B = H.take_blood(beaker,amount)
-			if (B)
-				beaker.reagents.reagent_list |= B
-				beaker.reagents.update_total()
-				beaker.on_reagent_change()
-				beaker.reagents.reconsider_reactions()
+			if(H.take_blood_legacy(beaker, amount) > 0)
 				update_icon()
 
 	if ((!valve_opened || tank.distribute_pressure == 0) && !breather && !attached)