Add a threading.run convenience function

[ZeroIntensity (Peter Bierma)]

@Rosuav

I’ll weigh in with some minor positive feedback. Not enough to outweigh the “not every three line function needs to be a builtin”, but an acknowledgement of the clunky ergonomics of threading.Thread

Note the word “every”. I agree that not every 3-line function needs to be in the stdlib, but I think it would be nice to have some 3-line functions in the stdlib, especially for super common operations.