Add working stub solutions to refactoring exercises:

[rbasso]

It was briefly discussed in #192,

I think we could keep those three exercises (beer-song, house, food-chain), but maybe we could take the boring part out of some of them.

They are about code refactoring, right?
Why not providing a working solution?!
We could also explain what the exercise is about in a HINTS.md file...

...that adding working stub solutions to the exercises that are essentially about refactoring could be a good idea.

I love the idea of providing the working solution and making it about refactoring.

👍 to a refactoring exercise (or a few).

This issue tracks the execution of this idea:

• beer-song beer-song: Rewrite tests and add working stub solution. #390
• house house: Fix tests to match x-common and add working stub solution. #348
• food-chain food-chain: Rewrite tests and add working stub solution. #346