Revise exercises default solution signatures

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 `class`es, which encapsulate no data).
Some concepts are used too often (`object`s).
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.

Review the repository's exercise ref-implementations, stubs, and tests against each exercise specification, starting with the existing solution concepts described in the issue. Identify more idiomatic defaults, update the implementations while preserving specifications, and document recommendations for future exercise design. Done means the reviewed exercises, stubs, and tests consistently use the chosen concepts and the recommendations are recorded.

Written by the indexing model from the issue text.