Improve quality of wallet example

[fubuloubu]

What's your issue about?

I was working on a black box tester for the examples and realized the wallet example is pretty difficult to understand (in my opinion). It may also suffer from some older syntax rules that might not work anymore as it was written 2 months ago. I think there is a lot of potential to show off some of the more obscure methods available in viper through this contract, so I think if someone just went and cleaned it up (and added a LOT of comments about why certain hard numbers are chosen), it would be very helpful.

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[fubuloubu]

Closing due to #356