bug#36597: 27.0.50; rehash hash tables eagerly in pdumper

[Eli Zaretskii]

> Cc: larsi@gnus.org, 36597-done@debbugs.gnu.org
> From: Paul Eggert <eggert@cs.ucla.edu>
> Date: Tue, 11 Aug 2020 10:31:24 -0700
> 
> The compilation issue was due to pdumper enabling -Wconversion, which causes 
> more trouble than it cures (but that is a different topic). I worked around 
> the 
> glitch by installing the attached further patch, which should also help 
> explain 
> the motivation for make_nil_vector.

Is it really worth it?  The code is now a kind of puzzle (what doe
those ALLOW/DISALLOW_IMPLICIT_CONVERSION macros do?), and Fmake_vector
is a very thin wrapper around the C function it calls.  So I think
this change is for the worse, sorry.