Do something with legacy skeleton datetime APIs

[sffc]

Currently they are behind the feature = "datagen" flag in icu_datetime. What should we do with them?

1. Leave as-is, potentially with a bit of documentation. Question: does feature = "datagen" mean unstable?
2. Move to icu::datetime::provider
3. Move to icu_provider_source, the only client
4. Make a new crate for it
5. Other suggestions?

@zbraniecki @robertbastian @Manishearth

[robertbastian]

I'd prefer 3 or 2. I think we should get rid of all the markers, so there's probably not much of an advantage of 2.

[Manishearth]

I agree, 3 or 2, preference for 3