Implements WidenLiteral in terms of Join helper

[ahrjarrett]

Happy to add additional tests or make adjustments. Also I'm not sure if I should inline Join, will that leak into the global namespace the way I declared it?

This alternate definition of WidenLiteral should be at parity with the current implementation. It works by distributing both types (the lower bound, and the upper bound), and choosing the upper member in the event of an intersection.

I named it join because that feels like the correct term, but I don't have a math background so I could be wrong.

[mattpocock]

Oddly I prefer the more explicit one, since it's dead-simple to understand.

[ahrjarrett]

IMO WidenLiteral is not dead-simple at all. In fact, I'd say it takes a pretty advanced understanding of how unions work in TS to appreciate the amount of implicit work it's doing.

I respect your right to close this PR, and won't press the issue. This quote comes to mind, it's one of my favorites, so I'll share it:

The purpose of abstraction is not to be vague, but to create a new semantic level in which one can be absolutely precise.

Best of luck!