A proposal to add 'flattening' support to the standard library for Set and List

e.g. ```flatten([1, 2], [3, 4]) == [1, 2, 3, 4]```

@ioanarucareanu and I both came across the fact that there is no library function to flatten a list of lists or a set of sets.
While trivial to implement, incorporating this in stdlib seems useful.