Added definition, exercise, and solution for Nat to Section 6.1

[Liable5607]

I believe that the structure of Nat isn't explained. However it is used in Section 6.1 for the definition of the dependent type example Vect and for many of the exercises in Chapter 6. So I added it and a small exercise using it (calculating the length of a List).

I'm very sorry if it was explained in the tutorial and I missed it. I checked a few times to make sure and didn't see it but I could have missed it.

[Liable5607]

I'm not sure what do about the build failure. It seems that it wasn't triggered by the code I wrote, e.g.

Error: While processing right hand side of ex4. Undefined name zipWith.

Error: While processing right hand side of replicate'. Undefined name replicate.

Perhaps I'm misintepreting the build messages though!

[stefan-hoeck]

By redefining Nat in this module, you create a lot of ambiguity that the compiler has no chance of resolving. All types mentioning Nat such as Vect but also (transitively) zipWith or replicate' can no longer be elaborated without ambiguity. The best solution would be to give Nat and its constructors a different name and inform readers about the correct names from the Prelude in the text.

[Liable5607]

Thank you for explaining my mistake!