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/checkpokerus.asm

@@ -0,0 +1,25 @@
+CheckPokerus: ; 4d860
+; Return carry if a monster in your party has Pokerus
+
+; Get number of monsters to iterate over
+	ld a, [PartyCount]
+	and a
+	jr z, .NoPokerus
+	ld b, a
+; Check each monster in the party for Pokerus
+	ld hl, PartyMon1PokerusStatus
+	ld de, PARTYMON_STRUCT_LENGTH
+.Check:
+	ld a, [hl]
+	and $0f ; only the bottom nybble is used
+	jr nz, .HasPokerus
+; Next PartyMon
+	add hl, de
+	dec b
+	jr nz, .Check
+.NoPokerus:
+	and a
+	ret
+.HasPokerus:
+	scf
+	ret