Unblock Datagen 1.3 by releasing Elsa or reverting to an older version of Elsa

[sffc]

Datagen depends on a git hash of Elsa because the test test_fallback_options uses an unreleased API. We have a few choices:

1. Release Elsa and update ICU4X's Cargo.toml to remove the git hash
   • Pro: We can use the new Elsa APIs
   • Con: The next Elsa release is not completely trivial because it seems it was discovered that some other newly added APIs are unsound (not the one we're using): FrozenMap unsound if Hash panics in such a way as to prevent HashMap rehashing to succeed. Manishearth/elsa#48
2. Change ICU4X to not use the new Elsa APIs (use a RwLock<HashMap> instead)
   • Pro: No external dependency
   • Con: Less pretty code
3. Depend on the git hash in dev-dependencies only
   • Pro: Unblocks the release with no code changes
   • Con: Multiple versions of Elsa in tree

@Manishearth @robertbastian

[Manishearth]

I'm planning on releasing a non breaking release, I just have a backlog of things to do on elsa. I thought I already communicated this; I was just busy that week. I can do this now and ignore the backlog.

[Manishearth]

newly added APIs are unsound

No, it's existing APIs, but it changes the model of the type and raises questions of how to best support other APIs, including the one we're using (though the one we're using can be made to work regardless, it's just potentially more work)

Oh if we're using into_tuple_vec that's fine

[Manishearth]

Elsa 1.9.0 published