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 18:45:19 +0200
> 
> Currently creating unibyte strings with the C module API is rather
> convoluted: one has to create a list with the individual bytes and then
> call `unibyte-string', requiring lots of memory allocations.  Given that
> creating a unibyte string is more fundamental than creating a multibyte
> string, I propose adding functions to create and extract unibyte strings
> that exactly mirror the existing support for multibyte strings.

Can you explain why you need to create unibyte strings, or describe a
use case where this would be required?