[nit] GADTs: add #require directives for abstracting code listings

[dijkstracula]

In the GADT chapter, the sum_file_sizes code makes use of Sys_unix.ls_dir and Core_unix.lstat. However, those modules are unbound for readers who are following along in the REPL.

utop # Sys_unix.ls_dir;;
Error: Unbound module Sys_unix
utop # Core_unix.lstat;;
Error: Unbound module Core_unix
Hint: Did you mean Core__Pid?
utop #

This patch simply adds a code listing #requireing them, and removes the TODO indicating that there perhaps ought to be such a code listing.

utop # #require "core_unix";;
utop # #require "core_unix.sys_unix";;

utop # Core_unix.lstat;;
- : string -> Core_unix.stats = <fun>
utop # Sys_unix.ls_dir;;
- : string -> string list = <fun>

Thank you for an enjoyable book.