Add an example about phantom types

[japaric]

I really liked the example about unit checking that showed up in reddit the other day.

This was also part of the cheatsheet that recently got removed in favor of rustbyexample.

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@mdinger is working on this. See #145

[mdinger]

How did I miss this? I thought I searched for this issue and didn't see one...

[japaric]

How did I miss this? I thought I searched for this issue and didn't see one...

Because I messed up and typed "shadow types" instead of "phantom types" when I created this issue. :P

I fixed the title after you opened #145. I think we'd better use #145 to track this. Closing.