Add support for 128bit numbers

[KodrAus]

Closes #236

This PR adds support for encoding/decoding i128 and u128 which were stabilized in rustc 1.26.0, and supported as of serde 1.0.60.

There's a new a disabled-by-default i128 feature flag in bincode, which enables support for 128bit numbers. If bincode is compiled on a version of Rust that supports 128bit numbers but doesn't have the i128 feature enabled then we'll return a custom error at runtime suggesting they enable the feature.

If there's anything you'd like changed let me know. Thanks!

[KodrAus]

cc @TyOverby @dtolnay

[KodrAus]

I could rejig the .travis.yml a bit so we just build without the feature, and test with it so there's less output to worry about? Would you be interested in pinning a specific version of Rust in CI so we don't accidentally bump it when making changes? I was testing these changes locally on the latest stable and 1.18.0?

[TyOverby]

This looks great! Thanks!

My only request would be adding a test that utilizes the full range of the 128 bit numbers.

[TyOverby]

I'm totally fine with the travis output spew as I rarely need to look at it :D

As for pinning the version, I don't know? It looks like serde tests with

• 1.13.0
• 1.15.0
• stable
• beta
• nightly

If you'd like, feel free to build these into this PR (in a new commit) and we can enable testing on these channels.

[KodrAus]

@TyOverby It looks like we need 1.18.0 for the pub(restricted) syntax, so I've added that to the CI.

[TyOverby]

Looks great, thanks for the contributions!