problem_format: add C++ templates

[Riolku]

Many templates have been floating around in the DMOJ community for validation and input handling in checkers. This commit aims to consolidate them. It has two main goals:

• Correct. Duh.

• Simple. Other templates that circulate, including the ones I have published, are too complex. People naively try and write their own. I am sick and tired of reading over incorrect validators.

  These templates forgo some principles of good design (such as object-oriented programming) in favour of pure simplicity. They should be simple enough that they are understandable by the broader community, and are not a black box. Hopefully this also dissuades re-writing.

[quantum5]

Can we make sure the templates pass clang-format?

[Riolku]

Yes, I'd be happy to add CI for that. Would you also like me to add some tests for the methods using ctest?

[quantum5]

Would you also like me to add some tests for the methods using ctest?

Sure, but at least make sure they build or something.