Added a new list exercise to Section 3.4

[Liable5607]

At the beginning of 6.iii, in the part discussing Vect concatenation, there was a reference to list concatenation:

It is possible to concatenate two Lists with the (++) operator. Surely, this should also be possible for Vect. But Vect is indexed by its length, so we have to reflect in the types exactly how the lengths of the inputs affect the lengths of the output. Here's how to do this:

I implemented that as an exercise (and checked it against Prelude.List.(++)) and thought that was interesting. It seemed to fit better in Section 3.iv though. I do think because there aren't a lot of List recursion exercises there, having another one is nice.

[stefan-hoeck]

Thanks. It's definitely good to have more basic exercises with recursion. Once people arrive at "Recursion and Folds", it would be another good exercise to re-implement many of the recursive functions from other chapters in a tail-recursive manner.