[WIP] relax Data.String.CodeUnits.slice + related changes as per #143

[Quelklef]

Description of the change

This PR closes #143.

That issue originally was about changing the bounds checking of Data.String.CodeUntis.slice. It has since been decided that the appropriate fix is to remove bounds checking altogether and to change the type from Int -> Int -> Maybe String to Int -> Int -> String.

Additionally, there is discussion in that ticket about improving similar string-and-index operations to be more consistent with the API. Hence the WIP-ness of this PR, so that those additional changes can be added as they are pointed out.

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Checklist:

• Added the change to the changelog's "Unreleased" section with a reference to this PR (e.g. "- Made a change (#0000)")
• Linked any existing issues or proposals that this pull request should close
• Updated or added relevant documentation
• Added a test for the contribution (if applicable)

[JordanMartinez]

🏓 @thomashoneyman

[Quelklef]

Hey, I think this is my very first open-source contribution! Cheers all 🥂

[garyb]

@Quelklef Congrats! 🎉 😄