Add jiff as a backend for Timestamp

[inklesspen]

The chrono maintainer has soft-deprecated chrono in favor of jiff.

It seems like it would be pretty straightforward to add jiff::Timestamp as an underlying type for serenity::…::Timestamp.

The only thing that might require discussion is what to do about the feature flags. I can see two possible approaches:

1. add a jiff feature flag, and use #[cfg(not(any(feature = "chrono", feature = "jiff"))] for the time backend.
2. add both jiff and time feature flags; since chrono is one of the default features anyway, this shouldn't be a huge hassle for users.

Although jiff isn't yet included in typesize, so maybe that would be an issue? I don't really understand what that is used for yet. But it should be pretty simple to add it if needed. I'll check with @GnomedDev on that.

I'd be happy to send a PR for this, if it'd be welcomed.

[inklesspen]

I implemented approach 1 in this commit: inklesspen@86af24c

I haven't opened a PR for it yet, though, because I think it's blocked on GnomedDev/typesize#10.

[GnomedDev]

I dislike the situation currently where we have different backends for time, since it means we have non-additive features and implementation complexity. I feel like, since the Rust ecosystem seems to be moving to Jiff anyway, we should just take a stand and only support it.