Add an example about phantom types

[mdinger]

I tried to add an example about Phantom types at mdinger@b03fc84 which were discussed on the mozilla blog. It definitely doesn't work now because I tried embedding 2 separate live code examples in a single page.

The usefulness of Phantom types is not immediately obvious to me so I thought a better example should be included. However, using that Add there can be confusing and I don't know where all the pieces of it are explained.

The first example became too complicated (long) when I added the second so I tried to split it. Any feedback is welcome. I cobbled it together using IRC for help.

I definitely don't know what you think about my list of cross references included but I tried to include references to the discussed topics anyway. No style was applied either.

[mdinger]

From #144 (comment): Reddit discussed this the other day. It is also mentioned in the old rust cheatsheet though Rust syntax looks a little different. The Haskell documentation for Phantom type is here.

[mdinger]

Examples will now build. Mostly just moved them around: mdinger@dd474e9

[japaric]

@mdinger Could you send a PR about this change? I can't really test this locally as it is, and that makes it hard for me to review.

[mdinger]

Done: #160

[mdinger]

Fixed by #160