bug#23487: 25.0.93; Modules: add functionality to create and copy unibyte strings

[Eli Zaretskii]

> From: Philipp Stephani <p.stephani2@gmail.com>
> Date: Mon, 09 May 2016 18:06:41 +0000
> Cc: 23487@debbugs.gnu.org
> 
> I don't have a concrete use case per se, but the API feels incomplete without 
> this functionality, especially given
> that creation of multibyte strings is supported and uses unibyte strings 
> under the hood.

Unibyte strings are very rarely used in Emacs core, and for a good
reason.  So I think we'd really want to see real-life use cases before
we decide to support them in modules.