[TM-317] Pure `UNPACK`

Description

After all, in hurry, I managed to merge the previous version of this MR to babylon :( That branch was removed right after this.

Related issue(s)

https://issues.serokell.io/issue/TM-317

✅ Checklist for your Merge Request

Related changes (conditional)

• Tests (see short guidelines)

  • If I added new functionality, I added tests covering it.
  • If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.

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  • I checked whether I should update the docs and did so if necessary:
    • README
    • Haddock
    • docs/

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• My code complies with the style guide.