factor out Fraction as a standalone consumable library

[erikerlandson]

My interest in this is totally self-serving - if there was a standalone lib then I would have the option of using it for exponents on coulomb 😃

Regardless, it seems like a type that is useful in and of itself, and maybe other projects would have use cases as well!

[zainab-ali]

It's a neat idea. I wonder what other numeric constructs could be promoted to the type level (intervals maybe?). I'll have a think on it.