[Decimals and Fractions]: Refine Concepts and Consider Adapting the Related Exercise from Julia

See discussion & proposed exercise [here](https://github.com/exercism/julia/pull/966).

See related concept write-up:

- [Fractions](https://github.com/exercism/python/tree/main/concepts/fractions)

We would need to also complete a `Decimals` concept and related concept exercise.
We might also want to contemplate more than one `Fractions` exercise, since the strategy of making and using Fractions changes when you need to inter-operate with other numeric types, or make fractions out of floating point numbers.