bug#29220: 26.0.90; eieio-persistent-read fail to restore saved object.

[Eli Zaretskii]

> From: Eric Abrahamsen <eric@ericabrahamsen.net>
> Cc: pierre.techoueyres@free.fr (Pierre Téchoueyres),
>   29220@debbugs.gnu.org,  Noam Postavsky <npostavs@users.sourceforge.net>,
>  John Wiegley <jwiegley@gmail.com>, Eli Zaretskii <eliz@gnu.org>
> Date: Tue, 05 Dec 2017 14:58:45 -0800
> 
> > I think so, but we're pretty late in the pretest, so better ask Eli&John
> > if they think it's OK for emacs-26.
> 
> Okay: John and Eli, this patch is to handle changes in object
> implementation in Emacs 26: they are now implemented with records, which
> can't be round-tripped with prin1 and read, they have to be written and
> read specially. The eieio-persistent mechanism handles that, but
> previously did not look inside hash tables or vectors to see if there
> were objects in there.
> 
> That's causing failure in the pcache library, which other libraries
> depend on. I'm hoping we can sneak this in under the wire so that pcache
> doesn't fail in Emacs 26.

Fine with me to put this on emacs-26.