Add simple examples for CodeGen

[showell]

As Rupert and I have been discussing on chat, it would be nice to have some really simple examples that people could look at.

I started with this package today, using good old "factorial":

https://package.elm-lang.org/packages/showell/elm-data-util/latest/

I recommend forking that package. (You're welcome to also directly link to it, but I may clutter it in future versions with things that aren't directly applicable to CodeGen.)

Of course, you're also welcome to just borrow some of the code, or just use it for inspiration.