Add optional serde support for Rational

[msrd0]

This PR allows (de)serializing rationals using serde. If the rational is representable as an integer, we serialize it as such, otherwise as a string displaying the fraction.

[ijagberg]

Thanks for your PR!

Would you mind adding a few unit tests? (De)serializing an integer, a non-integer, and very small/large values would be nice to see.

[msrd0]

Sure, I added some unit tests.

[ijagberg]

Great, thank you!

As I mentioned in the issue you posted, I have some qualms about changing the serialization behavior based on the value of the rational. It would seem surprising to me as a user of the library if Rational::new(1, 2) serialized to a string, but Rational::new(5, 1) serialized to an integer. Why not just always serialize to a string? Rational::new(5, 1) would become "5/1". That seems fine to me, and if I was writing something that was parsing these JSON values I would only need to handle JSON strings, and not worry about first checking if the value is an integer or not. This would also maybe help with the arbitrary precision stuff you wrote about?

We could still support deserializing from both integers and strings however, that seems like a reasonable feature to include.

[msrd0]

I changed it so that it always serializes as a string.

This would also maybe help with the arbitrary precision stuff you wrote about?

No, that's just serde_json being stupid in handling very big numbers. Which, given how loosely json defines what a "number" is, maybe shouldn't be surprising. But as I wrote, using serde_json::Value for the tests ended up working as expected.

[ijagberg]

Gotcha. Anyways, thanks for the contribution, I'll merge this and release it in a sec.

[ijagberg]

Released version 1.7.0