[m-rev.] for review: add_suffix

[Julien Fischer]

On Sun, 18 Oct 2020, Zoltan Somogyi wrote:

> For review by anyone.

> Add add_suffix to the standard library.
> 
> compiler/write_deps_file.m:
> library/string.m:
>     Move a generally-useful function to the library.
> 
> NEWS:
>     Announce the addition.

That's fine.

Julien.