Add autoload cookies in seq.el

[Tino Calancha]

Hi,

lisp/emacs-lisp/seq.el
lack of autoload cookies.

I see in the master branch that some files are starting
to use funtions from this library.
For instance,
* lisp/subr.el (read-multiple-choice)
* lisp/ibuffer.el (define-ibuffer-column name)
(it may be more cases, i didn't look in detail).

We should:
I) Add autoload cookies for the most useful functions
   defined in seq.el (as it was done in cl-seq).
II) Or add (require 'seq) in those files using such functions.