bug#43598: replace-in-string: finishing touches

[Eli Zaretskii]

> From: Mattias Engdegård <mattiase@acm.org>
> Date: Sun, 27 Sep 2020 18:14:36 +0200
> Cc: 43598-done@debbugs.gnu.org
> 
> Looks like we are done now. string-replace seems to be substantially faster 
> than replace-regexp-in-string.
> Good job!

Thanks.  Is it possible to have some speed comparison for these two?