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/math/sine.asm

@@ -0,0 +1,4 @@
+_Sine:: ; 84d9
+; a = d * sin(e * pi/32)
+	ld a, e
+	calc_sine_wave