refactor: Refine contract, use `for*/hash`
refactor: Refine contract, use `for*/hash`

We use a manually defined contract that still does not fully cover all
requirements. The built-in contract constructor `hash-of` is much better
suited for this use case.

`for*/hash` does the same as our current construct `for*/list` +
`list->string`, but is potentially more efficient.

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