Set up basic diplomat workflow

[Manishearth]

Progress on #104

This adds a simple diplomat integration, but does not use it yet. When we start using it we should add CI that runs cargo run -p diplomat-gen and ensures there is no diff.

[Manishearth]

I added a bunch of bindings! No tests yet, and I don't yet check in any bindings.

[jasonwilliams]

LGTM, I've ran cargo run -p diplomat-gen and seen the header files it generates. Looks like a good start. Ideally we may want a contributing guide to make sure the wrapper lib is updated alongside the underlying temporal_rs code but it sounds like some CI can be added for that. I also don't think that's a blocker for this.

[Manishearth]

@jasonwilliams yep, as I mention in the issue, my plan is to do that in future PRs. My goal is to get this in a place where collaboration can happen, we can add CI and proper tests later.

[nekevss]

This is looking great! Thanks for looking into this and helping out! Out of curiosity, have you been able to take a look at #169? If I'm understanding diplomat correctly, I don't think that it will affect any subsequent work related to diplomat, but I wanted to confirm first.

[Manishearth]

@nekevss it shouldn't matter, exposing it over FFI will be additional work but that's about it.

One thing that could be tricky is if you ever decided to expose data sources for calendars: currently we're able to cheaply clone Calendar everywhere (both temporal_rs and temporal_capi do this) because we know they're compiled-data backed, changing this might require wrapping them in an Arc or something.

[jedel1043]

@Manishearth How does ICU4X expose provider APIs? Because I was thinking we could change TimeZoneProvider to be a singleton DataProvider that returns some kind of tzif-like struct, which could make it much easier to expose through FFI if ICU4X already has a way to do that. And if we can do that, we can adapt that solution to also work with calendars.

[Manishearth]

We have _with_provider APIs over FFI that take a &DataProvider, which is not persistently borrowed: it returns payloads that contain Arc/Rc.

[Manishearth]

FWIW my preference is for this to be merged, I could push more APIs, and might do that on my flight tomorrow, but it will be nice to have something in.

[nekevss]

This looks good to me! 😄 Just needs a rebase / merge with main

[Manishearth]

Oh, I didn't realize those were mandatory. Done!