Support i128 and u128

[asomers]

It would be great if serde_yaml could support u128 and i128. I'm working on a PR and I've got it mostly working. However, the code would be much simpler if it didn't have to support compiler versions < 1.26.0. Are you ok with abandoning old compilers? It's not a blocker if you aren't, but it does complicate things.

[dtolnay]

Thanks for the upcoming PR! I would like to continue to support the same range of compiler versions.

[asomers]

Ok. In that case i128 support will be contingent on the consumer using a newer compiler and gated by a feature flag, just like in the bincode crate.