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/loadpushoam.asm

@@ -0,0 +1,21 @@
+LoadPushOAM:: ; 4031
+	ld c, hPushOAM - $ff00
+	ld b, .PushOAMEnd - .PushOAM
+	ld hl, .PushOAM
+.loop
+	ld a, [hli]
+	ld [$ff00+c], a
+	inc c
+	dec b
+	jr nz, .loop
+	ret
+
+.PushOAM: ; 403f
+	ld a, Sprites / $100
+	ld [rDMA], a
+	ld a, (SpritesEnd - Sprites) / 4 ; 40
+.pushoam_loop
+	dec a
+	jr nz, .pushoam_loop
+	ret
+.PushOAMEnd