Add some debug mode for detecting misusages at runtime

[akrzemi1]

Provide some debug mode, like when some custom macro like BOOST_OUTCOME_DEBUG_MODE is defined, where result and outcome at the cost of run-time size or efficiency track whether the failure status (has_value()) has been checked before the object is destroyed of unsafely accessed (with assume_value()).

[ned14]

I'm still not convinced that this actually useful in practice. I think people think it would be useful, without evidence. But then I do have two status bits free (I thought I only had one), and I want to keep a status bit free going into the future in case they improve language support for detecting failure to observe these objects. So I now see no reason not to use a bit for this, it could be useful I suppose.