[m-dev.] adding promise_only_solution

[Peter Schachte]

> > If/when it gets compiler support, then
> > existing code would continue to work, but new code wouldn't need messy
> > lambdas and packaging up of multiple outputs.
> 
> If/when it gets compiler support, we'll add 
> 
> 	:- pragma obsolete(promise_only_solution/1).
> 
> of course ;-)

My point was that if you put it in builtin.m now, if you later build
it into the compiler in a backward compatible way, then you can
quietly remove it from builtin.m and existing code won't need to be
rewritten.  I suppose that's true if you put it in std_util, too,
but in that case you're leaving a mysterious dependency on std_util.

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