Improve challenges with alternative solutions and hints

[LoicRiegel]

Hi, I tried to improve a few challenged

advanced-forward

• added an alternative solution (a hint was already mentioned about that)

extreme-self-casting and advanced-variadic-generics:

• added hints
• Removed "solution2.py" and moved the alternative solution proposed there as a comment in the corresponding "solution.py" file. This is more consistent, and the fact that those file exist is easy to miss for someone landing on the "solution.py" from the website

Hope you find those contributions useful :)

[laike9m]

Thanks Loïc for the PR! I'll take a look when I have time

[laike9m]

LGTM, thanks again!