standard bijections of Natural numbers

[Freek98]

Added proofs showing that the type of natural numbers is isomorphic to it's own sum and product.

• Showed that if we have a function $f : \mathbb N \to \mathbb N$, which is increasing and starts at 0. We can then partition $\mathbb N$ into the pieces $[f k , f(k+1) )$.
• Applying this fun fact to the double function, derived that $\mathbb N \cong \mathbb N + \mathbb N$
• Applying this fun fact to the function ($n \mapsto \frac{n (n+1)}{2} $), derived that $\mathbb N \cong \mathbb N \times \mathbb N$.
• There are also some small lemmas on the order of natural numbers and increasing functions.