Singletons + refactor of /datum/observ + refactor of /decl/ into /singleton/ (#15519)

code/modules/mining/mine_items.dm

@@ -36,7 +36,7 @@
 	origin_tech = list(TECH_MATERIAL = 1, TECH_ENGINEERING = 1)
 	attack_verb = list("hit", "pierced", "sliced", "attacked")
 	hitsound = 'sound/weapons/rapidslice.ogg'
-	var/drill_sound = /decl/sound_category/pickaxe_sound
+	var/drill_sound = /singleton/sound_category/pickaxe_sound
 	var/drill_verb = "excavating"
 	var/autodrill = 0 //pickaxes must be manually swung to mine, drills can mine rocks via bump
 	sharp = TRUE
@@ -45,7 +45,7 @@
 
 	var/excavation_amount = 40
 	var/wielded = FALSE
-	var/wield_sound = /decl/sound_category/generic_wield_sound
+	var/wield_sound = /singleton/sound_category/generic_wield_sound
 	var/unwield_sound = null
 	var/force_unwielded = 5.0
 	var/force_wielded = 15.0
@@ -351,7 +351,7 @@
 	edge = TRUE
 	drop_sound = 'sound/items/drop/shovel.ogg'
 	pickup_sound = 'sound/items/pickup/shovel.ogg'
-	usesound = /decl/sound_category/shovel_sound
+	usesound = /singleton/sound_category/shovel_sound
 
 /obj/item/shovel/spade
 	name = "spade"
@@ -1222,13 +1222,13 @@ var/list/total_extraction_beacons = list()
 		if(prob(25))
 			playsound(loc, 'sound/items/screwdriver.ogg', 20, TRUE)
 		else
-			playsound(loc, /decl/sound_category/pickaxe_sound, 20, TRUE)
+			playsound(loc, /singleton/sound_category/pickaxe_sound, 20, TRUE)
 
 		var/successfully_sculpted = FALSE
 		while(do_after(user, 2 SECONDS) && sculpture_process_check(choice, user))
 			if(times_carved <= 9)
 				times_carved++
-				playsound(loc, /decl/sound_category/pickaxe_sound, 20, TRUE)
+				playsound(loc, /singleton/sound_category/pickaxe_sound, 20, TRUE)
 				continue
 			successfully_sculpted = TRUE
 			break
@@ -1356,7 +1356,7 @@ var/list/total_extraction_beacons = list()
 /obj/structure/punching_bag/attack_hand(mob/user as mob)
 	user.setClickCooldown(DEFAULT_ATTACK_COOLDOWN)
 	flick("[icon_state]2", src)
-	playsound(get_turf(src), /decl/sound_category/swing_hit_sound, 25, 1, -1)
+	playsound(get_turf(src), /singleton/sound_category/swing_hit_sound, 25, 1, -1)
 
 /obj/structure/weightlifter
 	name = "weight machine"

code/modules/mining/mine_turf_types.dm

@@ -63,7 +63,7 @@
 	canSmoothWith = null
 	openspace_override_type = /turf/simulated/open/chasm/airless
 
-	footstep_sound = /decl/sound_category/asteroid_footstep
+	footstep_sound = /singleton/sound_category/asteroid_footstep
 
 /turf/unsimulated/floor/asteroid/basalt/get_smooth_underlay_icon(mutable_appearance/underlay_appearance, turf/asking_turf, adjacency_dir)
 	underlay_appearance.icon = icon
@@ -96,7 +96,7 @@
 	smooth = SMOOTH_MORE | SMOOTH_BORDER | SMOOTH_NO_CLEAR_ICON
 	base_icon = 'icons/turf/smooth/ash.dmi'
 	base_icon_state = "ash"
-	footstep_sound = /decl/sound_category/sand_footstep
+	footstep_sound = /singleton/sound_category/sand_footstep
 	does_footprint = TRUE
 	footprint_color = COLOR_ASH
 	track_distance = 6

code/modules/mining/mine_turfs.dm

@@ -418,8 +418,8 @@ var/list/mineral_can_smooth_with = list(
 	if(prob(25))
 		var/datum/reagents/R = new/datum/reagents(20)
 		R.my_atom = src
-		R.add_reagent(/decl/reagent/stone_dust,20)
-		var/datum/effect/effect/system/smoke_spread/chem/S = new /datum/effect/effect/system/smoke_spread/chem(/decl/reagent/stone_dust) // have to explicitly say the type to avoid issues with warnings
+		R.add_reagent(/singleton/reagent/stone_dust,20)
+		var/datum/effect/effect/system/smoke_spread/chem/S = new /datum/effect/effect/system/smoke_spread/chem(/singleton/reagent/stone_dust) // have to explicitly say the type to avoid issues with warnings
 		S.show_log = 0
 		S.set_up(R, 10, 0, src, 40)
 		S.start()
@@ -618,7 +618,7 @@ var/list/mineral_can_smooth_with = list(
 	var/dug = 0 //Increments by 1 everytime it's dug. 11 is the last integer that should ever be here.
 	var/digging
 	has_resources = 1
-	footstep_sound = /decl/sound_category/asteroid_footstep
+	footstep_sound = /singleton/sound_category/asteroid_footstep
 
 	roof_type = null