Optional NewType Validation

[davidroeca]

One proposed implementation of #415

This should work as indicated by the docstring as long as this would make sense to mypy.

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[gvanrossum]

I just have to say that this is not as easy as you think. I don't have time to elaborate, sorry.

[davidroeca]

Agreed that this likely touches a number of first and third-party libraries, and that NewType should be limited in scope. Figured this could at least be a point of discussion, and finally people have responded to my issue so I believe this pull request has done its work.