Remove all code from main.asm (some labeled INCBINs, like out-of-context graphics, are still present)

engine/routines/ stores isolated out-of-context routines as individual files. It might be preferable later to append them to their related engine/ files in unique little SECTIONs, relying on the linkerscript to place them appropriately; or some other organization method. In the meantime, they're now easily findable apart from main.asm's other content.

engine/routines/applypokerustick.asm

@@ -0,0 +1,26 @@
+ApplyPokerusTick: ; 13988
+; decreases all pokemon's pokerus counter by b. if the lower nybble reaches zero, the pokerus is cured.
+	ld hl, PartyMon1PokerusStatus ; PartyMon1 + MON_PKRS
+	ld a, [PartyCount]
+	and a
+	ret z ; make sure it's not wasting time on an empty party
+	ld c, a
+.loop
+	ld a, [hl]
+	and $f ; lower nybble is the number of days remaining
+	jr z, .next ; if already 0, skip
+	sub b ; subtract the number of days
+	jr nc, .ok ; max(result, 0)
+	xor a
+.ok
+	ld d, a ; back up this value because we need to preserve the strain (upper nybble)
+	ld a, [hl]
+	and $f0
+	add d
+	ld [hl], a ; this prevents a cured pokemon from recontracting pokerus
+.next
+	ld de, PARTYMON_STRUCT_LENGTH
+	add hl, de
+	dec c
+	jr nz, .loop
+	ret