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/checksave.asm

@@ -0,0 +1,20 @@
+CheckSave:: ; 4cffe
+	ld a, BANK(sCheckValue1) ; BANK(sCheckValue2)
+	call GetSRAMBank
+	ld a, [sCheckValue1]
+	ld b, a
+	ld a, [sCheckValue2]
+	ld c, a
+	call CloseSRAM
+	ld a, b
+	cp SAVE_CHECK_VALUE_1
+	jr nz, .ok
+	ld a, c
+	cp SAVE_CHECK_VALUE_2
+	jr nz, .ok
+	ld c, $1
+	ret
+
+.ok
+	ld c, $0
+	ret