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

[BethanyG]

See discussion & proposed exercise here.

See related concept write-up:

• 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.