Move icu_plurals::rules::reference to datagen?

[robertbastian]

Seems to be datagen-only code?

Both icu_plurals::rules and icu_datetime::pattern have parallel reference and runtime modules, presumably reference is not used at runtime?

[sffc]

@zbraniecki

[sffc]

In 2.0 I'm adding https://unicode-org.github.io/icu4x/rustdoc/icu_datetime/neo_pattern/struct.DateTimePattern.html which is a polished API around datetime::pattern::reference. I don't know if we need/want a public API around plurals::pattern::reference. I'm not super happy with it being doc(hidden).

[zbraniecki]

The reason I added reference is that it allows for canonical parse/edit/serialize loopback on the rules/patterns.

This is used by datagen but the target audience is wider - customers who want to manipulate a Plural Rule should be able to acquire this parser/serializer pair from icu_plurals. I'm ok for this to be behind a flag, but I'd be against moving it out of the respective crate.

[sffc]

@zbraniecki, do you want the whole module exported with all its (fairly deep) AST APIs and things, or would you be happy with a high-level API with just a few functions like parse and serialize?