decimal: replace try_from with FromStr impl

This PR touches files which potentially affect the outcome of the tests of an exercise. This will cause all students’ solutions to affected exercises to be re-tested.

If this PR does not affect the result of the test (or, for example, adds an edge case that is not worth rerunning all tests for), please add the following to the merge-commit message which will stops student’s tests from re-running. Please copy-paste to avoid typos.

[no important files changed]

For more information, refer to the documentation. If you are unsure whether to add the message or not, please ping @exercism/maintainers-admin in a comment. Thank you!

What PR is this? Looks like you just copied the “no important files changed” message without telling us what your change actually is.

Related to decimal: replace try_from with FromStr impl by Joshuaisikah · Pull Request #2168 · exercism/rust · GitHub, fixing decimal: replace homebuilt try_from with FromStr impl · Issue #1037 · exercism/rust · GitHub, an open issue five years old.

@senekor

Looks LLM-generated, I don’t want to review this.