Revise exercises default solution signatures

[eparovyshnaya]

Currently we have exercises ref-implemented and stubbed mostly by the following concepts

• class
• data class
• object
• public functions

without any particular scheme of picking a concept for a task.

Sometimes the choise is far from being language idiomatic or following design principles (for example, empty classes, which encapsulate no data). Some concepts are used too often (objects). Some others, like public functions, are rare and look underestimated.

That would be nice to review all the solution, question the core concept of each one, pick the most idiomatic and reimplement ref-impl/stob/test, keeping specification intact.

It also would be great to work out sort of recommendations for further choosing.