Add `coerced`

[ozkutuk]

Description of the change This PR adds coerced Iso. A motivating example for this addition is provided within the documentation. It should be noted that this not add an additional dependency, as safe-coerce is already depended on transitively through newtype.

---

Checklist:

• [x] Added the change to the changelog's "Unreleased" section with a link to this PR and your username
• [ ] Linked any existing issues or proposals that this pull request should close
• [ ] Updated or added relevant documentation in the README and/or documentation directory
• [ ] Added a test for the contribution (if applicable)

[thomashoneyman]

Would you mind adding this to the changelog? Thanks!