Port variable-length-quantity exercise.

[Javran]

Closes #959.

Few questions that I'll need help from maintainer:

• What is the version number of an exercise? I couldn't find any from problem spec repo.
• What should I put in config.json, I'm not sure about pretty much all fields in there.
• What do you use for formatting automation? Space-padding stuff and explicit import list is pretty much everywhere nowaways but I find those practices counterproductive so unless there's an automation I'll leave my own formatting style as is.

[Javran]

Thanks for your detailed feedback! I've addressed ~all~ most of them, please take a look.

I'd also love to be more consistent in terms of code formatting if there is a preferred formatting automation.

[sshine]

I'll get back to this in the weekend! :-)