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/emptyallsrambanks.asm

@@ -0,0 +1,19 @@
+EmptyAllSRAMBanks: ; 4cf1f
+	ld a, $0
+	call .EmptyBank
+	ld a, $1
+	call .EmptyBank
+	ld a, $2
+	call .EmptyBank
+	ld a, $3
+	call .EmptyBank
+	ret
+
+.EmptyBank: ; 4cf34
+	call GetSRAMBank
+	ld hl, SRAM_Begin
+	ld bc, SRAM_End - SRAM_Begin
+	xor a
+	call ByteFill
+	call CloseSRAM
+	ret