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/getsquareroot.asm

@@ -0,0 +1,32 @@
+NUM_SQUARE_ROOTS EQU 255
+
+GetSquareRoot: ; 13b87
+; Return the square root of de in b.
+
+; Rather than calculating the result, we take the index of the
+; first value in a table of squares that isn't lower than de.
+
+	ld hl, .Squares
+	ld b, 0
+.loop
+; Make sure we don't go past the end of the table.
+	inc b
+	ld a, b
+	cp NUM_SQUARE_ROOTS
+	ret z
+
+; Iterate over the table until b**2 >= de.
+	ld a, [hli]
+	sub e
+	ld a, [hli]
+	sbc d
+
+	jr c, .loop
+	ret
+
+.Squares: ; 13b98
+x = 1
+rept NUM_SQUARE_ROOTS
+	dw x * x
+x = x + 1
+endr