New datagen API

[robertbastian]

Outstanding questions:

• DatagenDriver::with_collations or DatagenDriver::with_addtional_collations?

[sffc]

Initial ideas:

• Create a new type that takes an IterableDynamicDataProvider and plumbs it to a DataExporter
• Possible name: DataExportDriver

[sffc]

In 2.0:

• DatagenProvider::set_cldr_source(&mut self, ...) -> &mut Self
• DataExportDriver::set_locales(&mut self, ...) -> &mut Self

But we want to do:

let driver = DataExportDriver::new().set_locales(...);

But we should prevent mutation:

let mut driver = DataExportDriver::new();
driver.set_locales();

// do some exporting

let driver = driver.with_keys();

// do some more exporting

• @sffc - Maybe we should make this be a real builder so that we can resolve CldrSet
• @robertbastian - We need SourceData for that
• @Manishearth - I agree with the goals. The precise solution seems like an okay way of achieving it.

[sffc]

For datagen driver:

• Okay to have ::new() because there is a reasonable default set of settings, and this is basically just an options bag; however, locales and keys must be explicitly set. If they are not, calling export should result in an Err.
• Default should be:
  • Unset locales and keys
  • Recommended segmenter models, with method with_recommended_segmenter_models(). In docs, say "at the moment, this is all models"
  • No additional collations
  • Default fallback mode (RecommendedForExporter)
• Silence the Clippy for missing Default impl because it is an unusable default

LGTM: @sffc @robertbastian (@Manishearth)