Clump.list

@fwbrasil I was not clever enough with generics to figure out how to do this for the general case, like the way `flatten` works for any type of collection (`Set`, `List`, etc.). However I think this will fulfill the majority of cases since people can combine it with traverse which returns a `Clump[List[T]]`

On naming: I originally called this `flatRun` to line up with `.map(...).flatten => .flatMap`, however I think `list` is a lot clearer and also reads better. I also think maybe `clump.get` might be a better name than `clump.run` because `clump.get` aligns well with `map.get` which returns an `Option`