Support i128 and u128

[e00E]

This PR adds support for 128 bit integers.

RON does not support 128 bit integers. This is undocumented. The Readme wrongly claims that the full Serde data model is supported.

[e00E]

We might want to use https://docs.serde.rs/serde/macro.serde_if_integer128.html. Thoughts?

Data formats that wish to support Rust compiler versions older than 1.26 (or targets that lack 128-bit integers) may place the i128 / u128 methods of their Serializer and Deserializer behind this macro.

Data formats that require a minimum Rust compiler version of at least 1.26, or do not target platforms that lack 128-bit integers, do not need to bother with this macro and may assume support for 128-bit integers.

It is a good idea to be more supportive when this is cheap so I am in favor of including it.

[bh2smith]

I just want to say that this would be a very good addition to the overall ecosystem and that it will become immediately valuable within our project;

gnosis/dex-services#822

Without it, we fear that we may have to implement our own serializations 🤮 !

Really nice clean fix by the way! Looks like these changes fit very naturally in here.

[bh2smith]

It appears these is another place that needs u128 support? I'm still getting an error when calling ron::de::from_bytes. Have not yet found the culprit.

[kvark]

Amazing work, thank you!
bors r+

[kvark]

@bh2smith are you able to use the git master for now? Or do you need a release soon?

[bors]

Timed out.