[m-dev.] for review: add promise_only_solution/1 to std_util.m

[Peter Schachte]

> +:- func promise_only_solution(pred(T)) = T.
> +:- mode promise_only_solution(pred(out) is cc_multi) = out is det.
> +:- mode promise_only_solution(pred(out) is cc_nondet) = out is semidet.

To be complete, shouldn't there be versions of promise_only_solution
for nondet and multi preds as well?  I know the implementation has to
be completely different, and so this is difficult to do, but it seems
worth at least documenting this restriction in the current
implementation.

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