Implement Ser+De for Saturating<T>

[jbethune]

This implementation is heavily inspired by the existing trait implentation for std::num::Wrapping<T>.

fix #2708

[jbethune]

std::num::Saturating<T> was added in Rust version 1.74 which is above the minimum supported version of Serde. How should we handle this?

[jbethune]

I've added the rustversion crate to version-gate the implementation so that it also compiles for older Rust versions.

[oli-obk]

We do version detection in build.rs: https://github.com/serde-rs/serde/blob/74d06708ddff495161187ea490c4616291216346/serde/build.rs#L64 Please use that scheme to register a new cfg and put your new additions behind such a cfg

[oli-obk]

(also once done, please rebase and squash your commits)

[jbethune]

(also once done, please rebase and squash your commits)

Done. Thanks for the clarifications!

[jbethune]

your use case where these impls are going to be needed.

I'm making a game for a virtual 16 bit architecture with number semantics that saturate at 2^16-1 because that's the maximum level you can achieve for different stats.

[jbethune]

@oli-obk I have updated the deserialization code so that it maps exceedingly large or small values to the MAX and MIN values of the target type.

I will squash and rebase (if needed) after I have an updated approval from you.

Thanks!

[jbethune]

@oli-obk I have incorporated your requests and suggestions and squashed everything into one commit

[oli-obk]

Thanks!

[dtolnay]

Thanks — published in serde 1.0.198.