Make allergen traits make proper use of bitflags

So, there's really no need to even have these exclude each other if you just change the way they apply. Also, this should fix any potential issues with species' default allergens being overridden.

code/modules/mob/living/carbon/human/species/station/traits_vr/neutral.dm

@@ -155,30 +155,30 @@
 
 
 //Allergen traits! Not available to any species with a base allergens var.
-/datum/trait/allergy_gluten
+/datum/trait/allergy
 	name = "Allergy: Gluten"
 	desc = "You're highly allergic to gluten proteins, which are found in most common grains."
 	cost = 0
 	custom_only = FALSE
-	var_changes = list("allergens" = 16)
-	excludes = list(/datum/trait/allergy_nuts,/datum/trait/allergy_soy)
+	var/allergen = 16
 
-/datum/trait/allergy_nuts
+/datum/trait/allergy/apply(var/datum/species/S,var/mob/living/carbon/human/H)
+	S.allergens |= allergen
+	..(S,H)
+
+/datum/trait/allergy/nuts
 	name = "Allergy: Nuts"
 	desc = "You're highly allergic to hard-shell seeds, such as peanuts."
 	cost = 0
 	custom_only = FALSE
-	var_changes = list("allergens" = 64)
-	excludes = list(/datum/trait/allergy_gluten,/datum/trait/allergy_soy)
+	allergen = 64
 
-/datum/trait/allergy_soy
+/datum/trait/allergy/soy
 	name = "Allergy: Soy"
 	desc = "You're highly allergic to soybeans, and some other kinds of bean."
 	cost = 0
 	custom_only = FALSE
-	var_changes = list("allergens" = 32)
-	excludes = list(/datum/trait/allergy_gluten,/datum/trait/allergy_nuts)
-
+	allergen = 32
 
 // Spicy Food Traits, from negative to positive.
 /datum/trait/spice_intolerance_extreme