Support is_human_readable

[matklad]

Serde has this nifty API: https://docs.rs/serde/1.0.118/serde/trait.Deserializer.html#method.is_human_readable

We should use this so that toml looks like

rational = "1/3"

rather than as

rational = [1, 3]

[cuviper]

If we could support deserializing either form, that sounds nice. However, I think it would be a breaking change for untagged enums, where someone expects it to fall through from a Ratio variant to a String variant.

Another option is to provide helpers for #[serde(with = "module")], like the ones we added in either.

[matklad]

I think we should treat this as a breaking change either way -- this is serialization format, so the other end might not even use Rust for deserialization.