bug#33939: 26.1; Avoid loading libs eagerly or unnecessarily in mhtml-mode/sgml-mode

[Eli Zaretskii]

> From: Leo Liu <sdl.web@gmail.com>
> Date: Tue, 01 Jan 2019 10:12:05 +0800
> 
> Switching from html-mode I have noticed mhtml-mode can take a second or
> two (unpleasant noticeable delay) to load on my 2013 macbook air. The
> following is a small patch that alleviate the problem. Is it safe for
> emacs-26?

I cannot establish whether these changes are safe for Emacs 26.2,
since they are not really trivial.  Perhaps if you told more about
each change, I could make up my mind.

Some of the questions I would like to be able to answer in order to
make the decision:

  . which of the 'require's you want to remove take the lion's share
    of the load time?
  . why was seq-mapcat used originally instead of mapcan, and what is
    the semantics of replacing the former by the latter?
  . why did you switch the order of eval-when-compile in sgml-mode.el?

Thanks.