[m-rev.] for review: Add string.append(uo, in, in) mode.

[Mark Brown]

On Thu, Oct 24, 2019 at 2:58 PM Peter Wang <novalazy at gmail.com> wrote:
>
> library/string.m:
>     Add string.append(uo, in, in) mode. The comment about it being
>     multi instead of semidet was written back when string.append was
>     implemented in terms of list.append.
>
>     Implement remove_suffix using the new procedure (more efficient).
>
>     Implement remove_suffix_if_present using remove_suffix
>     (more efficient).
>
>     Add comments about the argument orders of remove_suffix,
>     det_remove_suffix, remove_suffix_if_present.

You could add an equivalent comment to remove_prefix, etc. Otherwise
this looks fine.