Add flatten

[creade]

From "THAT OTHER FUNCTIONAL LIBRARY":

"Recursively descends through nested Collections and create a flat Collection of all of the leaves."

[sghill]

The difficulty here is around type-safety. It's easy if we just return collection of Object, but that doesn't follow suit with the rest of the library. We could also cast to a specific type, but that leaves the door open for a ClassCastException.