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/battle/unreferenced_getgen1trainerclassname.asm

@@ -0,0 +1,21 @@
+Unreferenced_GetGen1TrainerClassName: ; 50a28
+	ld hl, Gen1TrainerClassNames
+	ld a, [wTrainerClass]
+	dec a
+	ld c, a
+	ld b, 0
+	add hl, bc
+	add hl, bc
+	ld a, [hli]
+	ld h, [hl]
+	ld l, a
+	ld de, wStringBuffer1
+.copy
+	ld a, [hli]
+	ld [de], a
+	inc de
+	cp "@"
+	jr nz, .copy
+	ret
+
+INCLUDE "data/text/unused_gen1_trainer_names.asm"