Ship data for certain components directly on crates.io

[sffc]

Data for certain components, especially ones that aren't locale-dependent, can be shipped on crates.io to make integration processes easier. Examples:

1. icu_normalizer
2. icu_properties
3. icu_segmenter

CC @robertbastian @hsivonen

[robertbastian]

I'm in favour of this. A few points that we discussed on Slack that I want to record here:

• We will need to figure out a way to combine baked providers without using ForkByKeyProvider. This can be achieved by generating macros that add the DataProvider implementations, instead of generating a provider struct.
  • This macro could also be used to select locales, although datagen should be the primary interface for that
• We probably need to ship postcard data as well, unless we figure out a way to fork between AnyProvider and BufferProvider. Tbd whether to do this in the same crate behind a feature, or have 2x crates

[hsivonen]

Marking this U-gecko, because this would make it easier to allow IDNA in the upstream url crate be (optionally) backed by ICU4X such that the relevant normalization data (and data that #2850 will hopefully introduce) won't be duplicated with the rest of Gecko. (As opposed to carrying Gecko-specific patches to this effect.)

[robertbastian]

Closing this in favour of #2945 which is the canonical shipped-compile-time-data issue.