Organize the engine/ directory, director's cut

Cleaned up `engine/routines`, in favor of moving files into more
appropriate directories. predef-related routines are now in top-level
`engine`.
`rtc/delete_save_change_clock.asm` has been split into both
`menus/delete_save.asm` and `rtc/reset_password.asm`.

Made a new subdirectory:
* engine/math: Contains all generic math-related routines.

engine/menus/delete_save.asm

@@ -0,0 +1,36 @@
+_DeleteSaveData: ; 4d54c
+	farcall BlankScreen
+	ld b, SCGB_DIPLOMA
+	call GetSGBLayout
+	call LoadStandardFont
+	call LoadFontsExtra
+	ld de, MUSIC_MAIN_MENU
+	call PlayMusic
+	ld hl, .Text_ClearAllSaveData
+	call PrintText
+	ld hl, .NoYesMenuHeader
+	call CopyMenuHeader
+	call VerticalMenu
+	ret c
+	ld a, [wMenuCursorY]
+	cp $1
+	ret z
+	farcall EmptyAllSRAMBanks
+	ret
+
+.Text_ClearAllSaveData: ; 0x4d580
+	; Clear all save data?
+	text_jump UnknownText_0x1c564a
+	db "@"
+
+.NoYesMenuHeader: ; 0x4d585
+	db 0 ; flags
+	menu_coords 14, 7, SCREEN_WIDTH - 1, TEXTBOX_Y - 1
+	dw .MenuData
+	db 1 ; default option
+
+.MenuData: ; 0x4d58d
+	db STATICMENU_CURSOR | STATICMENU_NO_TOP_SPACING ; flags
+	db 2 ; items
+	db "NO@"
+	db "YES@"

engine/menus/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