Migrate the GYB template to Sourcery

[uraimo]

Currently, a GYB template is used to generate specific extensions for each integer type.
Even if it's quite easy to figure out how GYB templating works, migrating the template to Sourcery will make the template seem a little less esoteric.
The bash script that generates the classes will have to be updated too.

[darthpelo]

I will work also on this one, because after the Krzysztof's talk at the last #Pragma conference, I want to start to use in my company too :D

[uraimo]

Thanks!!! 👍👍