[m-dev.] adding promise_only_solution

[Peter Schachte]

> As I mentioned at the last Mercury meeting, I'm planning
> on adding `promise_only_solution' to the Mercury standard
> library before the upcoming release.  One thing I haven't
> figured out, though, is exactly where to put it.
> One possibility is a new module, called say `promise.m'.
> Another possibility is to add it to builtin.m.

Ultimately, I think a prettier (and more efficient) way to make such a
promise should become part of the language proper, so I'd argue it
should go in builtin.m.  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.

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