Sort exercises by slug before emitting template

[csaltachin]

Should fix #2149.

Note that comparing slugs lexicographically doesn't work because it yields "10" < "2", etc. Here we parse each slug into its numeric part (defaulting to 0) and its rightmost letter (if present) before comparing them as int * string tuples.
Maybe the letter could be instead parsed as a char option or directly as an int, but I hope it's not too convoluted overall.

[cuihtlauac]

Suggested change

	let slug = exercise.slug in
	let len = String.length slug in
	if len = 0 then (0, "")
	else
	match slug.[len - 1] with
	| 'A' .. 'Z' as c ->
	(parse_int (String.sub slug 0 (len - 1)), String.make 1 c)
	| _ -> (parse_int slug, "")
	Scanf.sscanf exercise.slug "%d%[A-Z]" (fun s c -> (s, c))

What do you think of something like that?

[csaltachin]

Wow, I didn't know sscanf was so versatile -- thanks for showing me. I'll change this and remove parse_int.
This could throw at build time if sscanf fails, but I don't imagine the slug format will change drastically in the future. We can always just update the format string + receiver function if so.

[cuihtlauac]

Thanks @csaltachin, this looks good to me