128-bit integer support

[asomers]

i128 and u128 can now be serialized and deserialized. Support is conditionalized on the disabled-by-default i128 feature flag, and requires rustc 1.26.0 or later. The minimum required version of Serde is now 1.0.60.

[asomers]

Sigh, Travis seems to be skipping CI for the entire PR, not just that one commit. I'll try to fix it.

[dtolnay]

What part of this makes it necessary to have an i128 feature in Cargo.toml? Can you do it without a feature?

[asomers]

I could remove the feature. The biggest downside I see is that consumers could accidentally write code that fails to compile with older compilers. But if they're using i128 anyway, then they already have that problem.

[dtolnay]

Users can always accidentally write code that fails to compile with older compilers, so I would prefer not to add a Cargo feature for that reason.

[asomers]

Ok, I'll remove it. BTW, the main reason I used a feature was just because bincode did. Though in bincode's case, the feature also pulls in another dependency.

[dtolnay]

Thanks. I landed this without the changes to serde_yaml::Value. Typically for the use cases of Value either 64 bits is good enough or they actually need arbitrary precision (similar to serde_json/arbitrary_precision).

[asomers]

Works for me. Value is only used when you don't know the schema ahead of time, right? In my case, I do. As long as I can deserialize into a u128, I'm happy.

[dtolnay]

Correct. Deserializing into u128 will support numbers up to 128 bits.

I published this change in 0.8.6.